Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

15 files changed, 4271 insertions, 0 deletions

diff --git a/Documentation/kbuild/Kconfig.recursion-issue-01 b/Documentation/kbuild/Kconfig.recursion-issue-01
new file mode 100644
index 000000000..e8877db04
--- /dev/null
+++ b/Documentation/kbuild/Kconfig.recursion-issue-01
@@ -0,0 +1,57 @@
+# Simple Kconfig recursive issue
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+#
+# Test with:
+#
+# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
+#
+# This Kconfig file has a simple recursive dependency issue. In order to
+# understand why this recursive dependency issue occurs lets consider what
+# Kconfig needs to address. We iterate over what Kconfig needs to address
+# by stepping through the questions it needs to address sequentially.
+#
+#  * What values are possible for CORE?
+#
+# CORE_BELL_A_ADVANCED selects CORE, which means that it influences the values
+# that are possible for CORE. So for example if CORE_BELL_A_ADVANCED is 'y',
+# CORE must be 'y' too.
+#
+#  * What influences CORE_BELL_A_ADVANCED ?
+#
+# As the name implies CORE_BELL_A_ADVANCED is an advanced feature of
+# CORE_BELL_A so naturally it depends on CORE_BELL_A. So if CORE_BELL_A is 'y'
+# we know CORE_BELL_A_ADVANCED can be 'y' too.
+#
+#   * What influences CORE_BELL_A ?
+#
+# CORE_BELL_A depends on CORE, so CORE influences CORE_BELL_A.
+#
+# But that is a problem, because this means that in order to determine
+# what values are possible for CORE we ended up needing to address questions
+# regarding possible values of CORE itself again. Answering the original
+# question of what are the possible values of CORE would make the kconfig
+# tools run in a loop. When this happens Kconfig exits and complains about
+# the "recursive dependency detected" error.
+#
+# Reading the Documentation/kbuild/Kconfig.recursion-issue-01 file it may be
+# obvious that an easy to solution to this problem should just be the removal
+# of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
+# since CORE_BELL_A depends on CORE. Recursive dependency issues are not always
+# so trivial to resolve, we provide another example below of practical
+# implications of this recursive issue where the solution is perhaps not so
+# easy to understand. Note that matching semantics on the dependency on
+# CORE also consist of a solution to this recursive problem.
+
+mainmenu "Simple example to demo kconfig recursive dependency issue"
+
+config CORE
+	tristate
+
+config CORE_BELL_A
+	tristate
+	depends on CORE
+
+config CORE_BELL_A_ADVANCED
+	tristate
+	depends on CORE_BELL_A
+	select CORE
diff --git a/Documentation/kbuild/Kconfig.recursion-issue-02 b/Documentation/kbuild/Kconfig.recursion-issue-02
new file mode 100644
index 000000000..df245fd76
--- /dev/null
+++ b/Documentation/kbuild/Kconfig.recursion-issue-02
@@ -0,0 +1,63 @@
+# Cumulative Kconfig recursive issue
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+#
+# Test with:
+#
+# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
+#
+# The recursive limitations with Kconfig has some non intuitive implications on
+# kconfig sematics which are documented here. One known practical implication
+# of the recursive limitation is that drivers cannot negate features from other
+# drivers if they share a common core requirement and use disjoint semantics to
+# annotate those requirements, ie, some drivers use "depends on" while others
+# use "select". For instance it means if a driver A and driver B share the same
+# core requirement, and one uses "select" while the other uses "depends on" to
+# annotate this, all features that driver A selects cannot now be negated by
+# driver B.
+#
+# A perhaps not so obvious implication of this is that, if semantics on these
+# core requirements are not carefully synced, as drivers evolve features
+# they select or depend on end up becoming shared requirements which cannot be
+# negated by other drivers.
+#
+# The example provided in Documentation/kbuild/Kconfig.recursion-issue-02
+# describes a simple driver core layout of example features a kernel might
+# have. Let's assume we have some CORE functionality, then the kernel has a
+# series of bells and whistles it desires to implement, its not so advanced so
+# it only supports bells at this time: CORE_BELL_A and CORE_BELL_B. If
+# CORE_BELL_A has some advanced feature CORE_BELL_A_ADVANCED which selects
+# CORE_BELL_A then CORE_BELL_A ends up becoming a common BELL feature which
+# other bells in the system cannot negate. The reason for this issue is
+# due to the disjoint use of semantics on expressing each bell's relationship
+# with CORE, one uses "depends on" while the other uses "select". Another
+# more important reason is that kconfig does not check for dependencies listed
+# under 'select' for a symbol, when such symbols are selected kconfig them
+# as mandatory required symbols. For more details on the heavy handed nature
+# of select refer to Documentation/kbuild/Kconfig.select-break
+#
+# To fix this the "depends on CORE" must be changed to "select CORE", or the
+# "select CORE" must be changed to "depends on CORE".
+#
+# For an example real world scenario issue refer to the attempt to remove
+# "select FW_LOADER" [0], in the end the simple alternative solution to this
+# problem consisted on matching semantics with newly introduced features.
+#
+# [0] https://lkml.kernel.org/r/1432241149-8762-1-git-send-email-mcgrof@do-not-panic.com
+
+mainmenu "Simple example to demo cumulative kconfig recursive dependency implication"
+
+config CORE
+	tristate
+
+config CORE_BELL_A
+	tristate
+	depends on CORE
+
+config CORE_BELL_A_ADVANCED
+	tristate
+	select CORE_BELL_A
+
+config CORE_BELL_B
+	tristate
+	depends on !CORE_BELL_A
+	select CORE
diff --git a/Documentation/kbuild/Kconfig.select-break b/Documentation/kbuild/Kconfig.select-break
new file mode 100644
index 000000000..365ceb342
--- /dev/null
+++ b/Documentation/kbuild/Kconfig.select-break
@@ -0,0 +1,33 @@
+# Select broken dependency issue
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+#
+# Test with:
+#
+# make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.select-break menuconfig
+#
+# kconfig will not complain and enable this layout for configuration. This is
+# currently a feature of kconfig, given select was designed to be heavy handed.
+# Kconfig currently does not check the list of symbols listed on a symbol's
+# "select" list, this is done on purpose to help load a set of known required
+# symbols. Because of this use of select should be used with caution. An
+# example of this issue is below.
+#
+# The option B and C are clearly contradicting with respect to A.
+# However, when A is set, C can be set as well because Kconfig does not
+# visit the dependencies of the select target (in this case B).  And since
+# Kconfig does not visit the dependencies, it breaks the dependencies of B
+# (!A).
+
+mainmenu "Simple example to demo kconfig select broken dependency issue"
+
+config A
+	bool "CONFIG A"
+
+config B
+	bool "CONFIG B"
+	depends on !A
+
+config C
+	bool "CONFIG C"
+	depends on A
+	select B
diff --git a/Documentation/kbuild/gcc-plugins.rst b/Documentation/kbuild/gcc-plugins.rst
new file mode 100644
index 000000000..3349966f2
--- /dev/null
+++ b/Documentation/kbuild/gcc-plugins.rst
@@ -0,0 +1,92 @@
+=========================
+GCC plugin infrastructure
+=========================
+
+
+Introduction
+============
+
+GCC plugins are loadable modules that provide extra features to the
+compiler [1]_. They are useful for runtime instrumentation and static analysis.
+We can analyse, change and add further code during compilation via
+callbacks [2]_, GIMPLE [3]_, IPA [4]_ and RTL passes [5]_.
+
+The GCC plugin infrastructure of the kernel supports building out-of-tree
+modules, cross-compilation and building in a separate directory.
+Plugin source files have to be compilable by a C++ compiler.
+
+Currently the GCC plugin infrastructure supports only some architectures.
+Grep "select HAVE_GCC_PLUGINS" to find out which architectures support
+GCC plugins.
+
+This infrastructure was ported from grsecurity [6]_ and PaX [7]_.
+
+--
+
+.. [1] https://gcc.gnu.org/onlinedocs/gccint/Plugins.html
+.. [2] https://gcc.gnu.org/onlinedocs/gccint/Plugin-API.html#Plugin-API
+.. [3] https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html
+.. [4] https://gcc.gnu.org/onlinedocs/gccint/IPA.html
+.. [5] https://gcc.gnu.org/onlinedocs/gccint/RTL.html
+.. [6] https://grsecurity.net/
+.. [7] https://pax.grsecurity.net/
+
+
+Files
+=====
+
+**$(src)/scripts/gcc-plugins**
+
+	This is the directory of the GCC plugins.
+
+**$(src)/scripts/gcc-plugins/gcc-common.h**
+
+	This is a compatibility header for GCC plugins.
+	It should be always included instead of individual gcc headers.
+
+**$(src)/scripts/gcc-plugins/gcc-generate-gimple-pass.h,
+$(src)/scripts/gcc-plugins/gcc-generate-ipa-pass.h,
+$(src)/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h,
+$(src)/scripts/gcc-plugins/gcc-generate-rtl-pass.h**
+
+	These headers automatically generate the registration structures for
+	GIMPLE, SIMPLE_IPA, IPA and RTL passes.
+	They should be preferred to creating the structures by hand.
+
+
+Usage
+=====
+
+You must install the gcc plugin headers for your gcc version,
+e.g., on Ubuntu for gcc-10::
+
+	apt-get install gcc-10-plugin-dev
+
+Or on Fedora::
+
+	dnf install gcc-plugin-devel
+
+Enable the GCC plugin infrastructure and some plugin(s) you want to use
+in the kernel config::
+
+	CONFIG_GCC_PLUGINS=y
+	CONFIG_GCC_PLUGIN_CYC_COMPLEXITY=y
+	CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y
+	...
+
+To compile the minimum tool set including the plugin(s)::
+
+	make scripts
+
+or just run the kernel make and compile the whole kernel with
+the cyclomatic complexity GCC plugin.
+
+
+4. How to add a new GCC plugin
+==============================
+
+The GCC plugins are in scripts/gcc-plugins/. You need to put plugin source files
+right under scripts/gcc-plugins/. Creating subdirectories is not supported.
+It must be added to scripts/gcc-plugins/Makefile, scripts/Makefile.gcc-plugins
+and a relevant Kconfig file.
+See the cyc_complexity_plugin.c (CONFIG_GCC_PLUGIN_CYC_COMPLEXITY) GCC plugin.
diff --git a/Documentation/kbuild/headers_install.rst b/Documentation/kbuild/headers_install.rst
new file mode 100644
index 000000000..f6c6b74a6
--- /dev/null
+++ b/Documentation/kbuild/headers_install.rst
@@ -0,0 +1,44 @@
+=============================================
+Exporting kernel headers for use by userspace
+=============================================
+
+The "make headers_install" command exports the kernel's header files in a
+form suitable for use by userspace programs.
+
+The linux kernel's exported header files describe the API for user space
+programs attempting to use kernel services.  These kernel header files are
+used by the system's C library (such as glibc or uClibc) to define available
+system calls, as well as constants and structures to be used with these
+system calls.  The C library's header files include the kernel header files
+from the "linux" subdirectory.  The system's libc headers are usually
+installed at the default location /usr/include and the kernel headers in
+subdirectories under that (most notably /usr/include/linux and
+/usr/include/asm).
+
+Kernel headers are backwards compatible, but not forwards compatible.  This
+means that a program built against a C library using older kernel headers
+should run on a newer kernel (although it may not have access to new
+features), but a program built against newer kernel headers may not work on an
+older kernel.
+
+The "make headers_install" command can be run in the top level directory of the
+kernel source code (or using a standard out-of-tree build).  It takes two
+optional arguments::
+
+  make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr
+
+ARCH indicates which architecture to produce headers for, and defaults to the
+current architecture.  The linux/asm directory of the exported kernel headers
+is platform-specific, to see a complete list of supported architectures use
+the command::
+
+  ls -d include/asm-* | sed 's/.*-//'
+
+INSTALL_HDR_PATH indicates where to install the headers. It defaults to
+"./usr".
+
+An 'include' directory is automatically created inside INSTALL_HDR_PATH and
+headers are installed in 'INSTALL_HDR_PATH/include'.
+
+The kernel header export infrastructure is maintained by David Woodhouse
+<dwmw2@infradead.org>.
diff --git a/Documentation/kbuild/index.rst b/Documentation/kbuild/index.rst
new file mode 100644
index 000000000..cee2f99f7
--- /dev/null
+++ b/Documentation/kbuild/index.rst
@@ -0,0 +1,30 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================
+Kernel Build System
+===================
+
+.. toctree::
+    :maxdepth: 1
+
+    kconfig-language
+    kconfig-macro-language
+
+    kbuild
+    kconfig
+    makefiles
+    modules
+
+    headers_install
+
+    issues
+    reproducible-builds
+    gcc-plugins
+    llvm
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/kbuild/issues.rst b/Documentation/kbuild/issues.rst
new file mode 100644
index 000000000..bdab01f73
--- /dev/null
+++ b/Documentation/kbuild/issues.rst
@@ -0,0 +1,15 @@
+================
+Recursion issues
+================
+
+issue #1
+--------
+
+.. literalinclude:: Kconfig.recursion-issue-01
+   :language: kconfig
+
+issue #2
+--------
+
+.. literalinclude:: Kconfig.recursion-issue-02
+   :language: kconfig
diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
new file mode 100644
index 000000000..2d1fc03d3
--- /dev/null
+++ b/Documentation/kbuild/kbuild.rst
@@ -0,0 +1,269 @@
+======
+Kbuild
+======
+
+
+Output files
+============
+
+modules.order
+-------------
+This file records the order in which modules appear in Makefiles. This
+is used by modprobe to deterministically resolve aliases that match
+multiple modules.
+
+modules.builtin
+---------------
+This file lists all modules that are built into the kernel. This is used
+by modprobe to not fail when trying to load something builtin.
+
+modules.builtin.modinfo
+-----------------------
+This file contains modinfo from all modules that are built into the kernel.
+Unlike modinfo of a separate module, all fields are prefixed with module name.
+
+
+Environment variables
+=====================
+
+KCPPFLAGS
+---------
+Additional options to pass when preprocessing. The preprocessing options
+will be used in all cases where kbuild does preprocessing including
+building C files and assembler files.
+
+KAFLAGS
+-------
+Additional options to the assembler (for built-in and modules).
+
+AFLAGS_MODULE
+-------------
+Additional assembler options for modules.
+
+AFLAGS_KERNEL
+-------------
+Additional assembler options for built-in.
+
+KCFLAGS
+-------
+Additional options to the C compiler (for built-in and modules).
+
+CFLAGS_KERNEL
+-------------
+Additional options for $(CC) when used to compile
+code that is compiled as built-in.
+
+CFLAGS_MODULE
+-------------
+Additional module specific options to use for $(CC).
+
+LDFLAGS_MODULE
+--------------
+Additional options used for $(LD) when linking modules.
+
+HOSTCFLAGS
+----------
+Additional flags to be passed to $(HOSTCC) when building host programs.
+
+HOSTCXXFLAGS
+------------
+Additional flags to be passed to $(HOSTCXX) when building host programs.
+
+HOSTLDFLAGS
+-----------
+Additional flags to be passed when linking host programs.
+
+HOSTLDLIBS
+----------
+Additional libraries to link against when building host programs.
+
+KBUILD_KCONFIG
+--------------
+Set the top-level Kconfig file to the value of this environment
+variable.  The default name is "Kconfig".
+
+KBUILD_VERBOSE
+--------------
+Set the kbuild verbosity. Can be assigned same values as "V=...".
+
+See make help for the full list.
+
+Setting "V=..." takes precedence over KBUILD_VERBOSE.
+
+KBUILD_EXTMOD
+-------------
+Set the directory to look for the kernel source when building external
+modules.
+
+Setting "M=..." takes precedence over KBUILD_EXTMOD.
+
+KBUILD_OUTPUT
+-------------
+Specify the output directory when building the kernel.
+
+The output directory can also be specified using "O=...".
+
+Setting "O=..." takes precedence over KBUILD_OUTPUT.
+
+KBUILD_EXTRA_WARN
+-----------------
+Specify the extra build checks. The same value can be assigned by passing
+W=... from the command line.
+
+See `make help` for the list of the supported values.
+
+Setting "W=..." takes precedence over KBUILD_EXTRA_WARN.
+
+KBUILD_DEBARCH
+--------------
+For the deb-pkg target, allows overriding the normal heuristics deployed by
+deb-pkg. Normally deb-pkg attempts to guess the right architecture based on
+the UTS_MACHINE variable, and on some architectures also the kernel config.
+The value of KBUILD_DEBARCH is assumed (not checked) to be a valid Debian
+architecture.
+
+ARCH
+----
+Set ARCH to the architecture to be built.
+
+In most cases the name of the architecture is the same as the
+directory name found in the arch/ directory.
+
+But some architectures such as x86 and sparc have aliases.
+
+- x86: i386 for 32 bit, x86_64 for 64 bit
+- sh: sh for 32 bit, sh64 for 64 bit
+- sparc: sparc32 for 32 bit, sparc64 for 64 bit
+
+CROSS_COMPILE
+-------------
+Specify an optional fixed part of the binutils filename.
+CROSS_COMPILE can be a part of the filename or the full path.
+
+CROSS_COMPILE is also used for ccache in some setups.
+
+CF
+--
+Additional options for sparse.
+
+CF is often used on the command-line like this::
+
+    make CF=-Wbitwise C=2
+
+INSTALL_PATH
+------------
+INSTALL_PATH specifies where to place the updated kernel and system map
+images. Default is /boot, but you can set it to other values.
+
+INSTALLKERNEL
+-------------
+Install script called when using "make install".
+The default name is "installkernel".
+
+The script will be called with the following arguments:
+
+   - $1 - kernel version
+   - $2 - kernel image file
+   - $3 - kernel map file
+   - $4 - default install path (use root directory if blank)
+
+The implementation of "make install" is architecture specific
+and it may differ from the above.
+
+INSTALLKERNEL is provided to enable the possibility to
+specify a custom installer when cross compiling a kernel.
+
+MODLIB
+------
+Specify where to install modules.
+The default value is::
+
+     $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE)
+
+The value can be overridden in which case the default value is ignored.
+
+INSTALL_MOD_PATH
+----------------
+INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory
+relocations required by build roots.  This is not defined in the
+makefile but the argument can be passed to make if needed.
+
+INSTALL_MOD_STRIP
+-----------------
+INSTALL_MOD_STRIP, if defined, will cause modules to be
+stripped after they are installed.  If INSTALL_MOD_STRIP is '1', then
+the default option --strip-debug will be used.  Otherwise,
+INSTALL_MOD_STRIP value will be used as the options to the strip command.
+
+INSTALL_HDR_PATH
+----------------
+INSTALL_HDR_PATH specifies where to install user space headers when
+executing "make headers_*".
+
+The default value is::
+
+    $(objtree)/usr
+
+$(objtree) is the directory where output files are saved.
+The output directory is often set using "O=..." on the commandline.
+
+The value can be overridden in which case the default value is ignored.
+
+KBUILD_ABS_SRCTREE
+--------------------------------------------------
+Kbuild uses a relative path to point to the tree when possible. For instance,
+when building in the source tree, the source tree path is '.'
+
+Setting this flag requests Kbuild to use absolute path to the source tree.
+There are some useful cases to do so, like when generating tag files with
+absolute path entries etc.
+
+KBUILD_SIGN_PIN
+---------------
+This variable allows a passphrase or PIN to be passed to the sign-file
+utility when signing kernel modules, if the private key requires such.
+
+KBUILD_MODPOST_WARN
+-------------------
+KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined
+symbols in the final module linking stage. It changes such errors
+into warnings.
+
+KBUILD_MODPOST_NOFINAL
+----------------------
+KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules.
+This is solely useful to speed up test compiles.
+
+KBUILD_EXTRA_SYMBOLS
+--------------------
+For modules that use symbols from other modules.
+See more details in modules.rst.
+
+ALLSOURCE_ARCHS
+---------------
+For tags/TAGS/cscope targets, you can specify more than one arch
+to be included in the databases, separated by blank space. E.g.::
+
+    $ make ALLSOURCE_ARCHS="x86 mips arm" tags
+
+To get all available archs you can also specify all. E.g.::
+
+    $ make ALLSOURCE_ARCHS=all tags
+
+KBUILD_BUILD_TIMESTAMP
+----------------------
+Setting this to a date string overrides the timestamp used in the
+UTS_VERSION definition (uname -v in the running kernel). The value has to
+be a string that can be passed to date -d. The default value
+is the output of the date command at one point during build.
+
+KBUILD_BUILD_USER, KBUILD_BUILD_HOST
+------------------------------------
+These two variables allow to override the user@host string displayed during
+boot and in /proc/version. The default value is the output of the commands
+whoami and host, respectively.
+
+LLVM
+----
+If this variable is set to 1, Kbuild will use Clang and LLVM utilities instead
+of GCC and GNU binutils to build the kernel.
diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
new file mode 100644
index 000000000..1cf1aebdd
--- /dev/null
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -0,0 +1,720 @@
+================
+Kconfig Language
+================
+
+Introduction
+------------
+
+The configuration database is a collection of configuration options
+organized in a tree structure::
+
+	+- Code maturity level options
+	|  +- Prompt for development and/or incomplete code/drivers
+	+- General setup
+	|  +- Networking support
+	|  +- System V IPC
+	|  +- BSD Process Accounting
+	|  +- Sysctl support
+	+- Loadable module support
+	|  +- Enable loadable module support
+	|     +- Set version information on all module symbols
+	|     +- Kernel module loader
+	+- ...
+
+Every entry has its own dependencies. These dependencies are used
+to determine the visibility of an entry. Any child entry is only
+visible if its parent entry is also visible.
+
+Menu entries
+------------
+
+Most entries define a config option; all other entries help to organize
+them. A single configuration option is defined like this::
+
+  config MODVERSIONS
+	bool "Set version information on all module symbols"
+	depends on MODULES
+	help
+	  Usually, modules have to be recompiled whenever you switch to a new
+	  kernel.  ...
+
+Every line starts with a key word and can be followed by multiple
+arguments.  "config" starts a new config entry. The following lines
+define attributes for this config option. Attributes can be the type of
+the config option, input prompt, dependencies, help text and default
+values. A config option can be defined multiple times with the same
+name, but every definition can have only a single input prompt and the
+type must not conflict.
+
+Menu attributes
+---------------
+
+A menu entry can have a number of attributes. Not all of them are
+applicable everywhere (see syntax).
+
+- type definition: "bool"/"tristate"/"string"/"hex"/"int"
+
+  Every config option must have a type. There are only two basic types:
+  tristate and string; the other types are based on these two. The type
+  definition optionally accepts an input prompt, so these two examples
+  are equivalent::
+
+	bool "Networking support"
+
+  and::
+
+	bool
+	prompt "Networking support"
+
+- input prompt: "prompt" <prompt> ["if" <expr>]
+
+  Every menu entry can have at most one prompt, which is used to display
+  to the user. Optionally dependencies only for this prompt can be added
+  with "if".
+
+- default value: "default" <expr> ["if" <expr>]
+
+  A config option can have any number of default values. If multiple
+  default values are visible, only the first defined one is active.
+  Default values are not limited to the menu entry where they are
+  defined. This means the default can be defined somewhere else or be
+  overridden by an earlier definition.
+  The default value is only assigned to the config symbol if no other
+  value was set by the user (via the input prompt above). If an input
+  prompt is visible the default value is presented to the user and can
+  be overridden by him.
+  Optionally, dependencies only for this default value can be added with
+  "if".
+
+ The default value deliberately defaults to 'n' in order to avoid bloating the
+ build. With few exceptions, new config options should not change this. The
+ intent is for "make oldconfig" to add as little as possible to the config from
+ release to release.
+
+ Note:
+	Things that merit "default y/m" include:
+
+	a) A new Kconfig option for something that used to always be built
+	   should be "default y".
+
+	b) A new gatekeeping Kconfig option that hides/shows other Kconfig
+	   options (but does not generate any code of its own), should be
+	   "default y" so people will see those other options.
+
+	c) Sub-driver behavior or similar options for a driver that is
+	   "default n". This allows you to provide sane defaults.
+
+	d) Hardware or infrastructure that everybody expects, such as CONFIG_NET
+	   or CONFIG_BLOCK. These are rare exceptions.
+
+- type definition + default value::
+
+	"def_bool"/"def_tristate" <expr> ["if" <expr>]
+
+  This is a shorthand notation for a type definition plus a value.
+  Optionally dependencies for this default value can be added with "if".
+
+- dependencies: "depends on" <expr>
+
+  This defines a dependency for this menu entry. If multiple
+  dependencies are defined, they are connected with '&&'. Dependencies
+  are applied to all other options within this menu entry (which also
+  accept an "if" expression), so these two examples are equivalent::
+
+	bool "foo" if BAR
+	default y if BAR
+
+  and::
+
+	depends on BAR
+	bool "foo"
+	default y
+
+- reverse dependencies: "select" <symbol> ["if" <expr>]
+
+  While normal dependencies reduce the upper limit of a symbol (see
+  below), reverse dependencies can be used to force a lower limit of
+  another symbol. The value of the current menu symbol is used as the
+  minimal value <symbol> can be set to. If <symbol> is selected multiple
+  times, the limit is set to the largest selection.
+  Reverse dependencies can only be used with boolean or tristate
+  symbols.
+
+  Note:
+	select should be used with care. select will force
+	a symbol to a value without visiting the dependencies.
+	By abusing select you are able to select a symbol FOO even
+	if FOO depends on BAR that is not set.
+	In general use select only for non-visible symbols
+	(no prompts anywhere) and for symbols with no dependencies.
+	That will limit the usefulness but on the other hand avoid
+	the illegal configurations all over.
+
+- weak reverse dependencies: "imply" <symbol> ["if" <expr>]
+
+  This is similar to "select" as it enforces a lower limit on another
+  symbol except that the "implied" symbol's value may still be set to n
+  from a direct dependency or with a visible prompt.
+
+  Given the following example::
+
+    config FOO
+	tristate "foo"
+	imply BAZ
+
+    config BAZ
+	tristate "baz"
+	depends on BAR
+
+  The following values are possible:
+
+	===		===		=============	==============
+	FOO		BAR		BAZ's default	choice for BAZ
+	===		===		=============	==============
+	n		y		n		N/m/y
+	m		y		m		M/y/n
+	y		y		y		Y/m/n
+	n		m		n		N/m
+	m		m		m		M/n
+	y		m		n		M/n
+	y		n		*		N
+	===		===		=============	==============
+
+  This is useful e.g. with multiple drivers that want to indicate their
+  ability to hook into a secondary subsystem while allowing the user to
+  configure that subsystem out without also having to unset these drivers.
+
+  Note: If the combination of FOO=y and BAR=m causes a link error,
+  you can guard the function call with IS_REACHABLE()::
+
+	foo_init()
+	{
+		if (IS_REACHABLE(CONFIG_BAZ))
+			baz_register(&foo);
+		...
+	}
+
+  Note: If the feature provided by BAZ is highly desirable for FOO,
+  FOO should imply not only BAZ, but also its dependency BAR::
+
+    config FOO
+	tristate "foo"
+	imply BAR
+	imply BAZ
+
+- limiting menu display: "visible if" <expr>
+
+  This attribute is only applicable to menu blocks, if the condition is
+  false, the menu block is not displayed to the user (the symbols
+  contained there can still be selected by other symbols, though). It is
+  similar to a conditional "prompt" attribute for individual menu
+  entries. Default value of "visible" is true.
+
+- numerical ranges: "range" <symbol> <symbol> ["if" <expr>]
+
+  This allows to limit the range of possible input values for int
+  and hex symbols. The user can only input a value which is larger than
+  or equal to the first symbol and smaller than or equal to the second
+  symbol.
+
+- help text: "help"
+
+  This defines a help text. The end of the help text is determined by
+  the indentation level, this means it ends at the first line which has
+  a smaller indentation than the first line of the help text.
+
+- misc options: "option" <symbol>[=<value>]
+
+  Various less common options can be defined via this option syntax,
+  which can modify the behaviour of the menu entry and its config
+  symbol. These options are currently possible:
+
+  - "defconfig_list"
+    This declares a list of default entries which can be used when
+    looking for the default configuration (which is used when the main
+    .config doesn't exists yet.)
+
+  - "modules"
+    This declares the symbol to be used as the MODULES symbol, which
+    enables the third modular state for all config symbols.
+    At most one symbol may have the "modules" option set.
+
+  - "allnoconfig_y"
+    This declares the symbol as one that should have the value y when
+    using "allnoconfig". Used for symbols that hide other symbols.
+
+Menu dependencies
+-----------------
+
+Dependencies define the visibility of a menu entry and can also reduce
+the input range of tristate symbols. The tristate logic used in the
+expressions uses one more state than normal boolean logic to express the
+module state. Dependency expressions have the following syntax::
+
+  <expr> ::= <symbol>                           (1)
+           <symbol> '=' <symbol>                (2)
+           <symbol> '!=' <symbol>               (3)
+           <symbol1> '<' <symbol2>              (4)
+           <symbol1> '>' <symbol2>              (4)
+           <symbol1> '<=' <symbol2>             (4)
+           <symbol1> '>=' <symbol2>             (4)
+           '(' <expr> ')'                       (5)
+           '!' <expr>                           (6)
+           <expr> '&&' <expr>                   (7)
+           <expr> '||' <expr>                   (8)
+
+Expressions are listed in decreasing order of precedence.
+
+(1) Convert the symbol into an expression. Boolean and tristate symbols
+    are simply converted into the respective expression values. All
+    other symbol types result in 'n'.
+(2) If the values of both symbols are equal, it returns 'y',
+    otherwise 'n'.
+(3) If the values of both symbols are equal, it returns 'n',
+    otherwise 'y'.
+(4) If value of <symbol1> is respectively lower, greater, lower-or-equal,
+    or greater-or-equal than value of <symbol2>, it returns 'y',
+    otherwise 'n'.
+(5) Returns the value of the expression. Used to override precedence.
+(6) Returns the result of (2-/expr/).
+(7) Returns the result of min(/expr/, /expr/).
+(8) Returns the result of max(/expr/, /expr/).
+
+An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
+respectively for calculations). A menu entry becomes visible when its
+expression evaluates to 'm' or 'y'.
+
+There are two types of symbols: constant and non-constant symbols.
+Non-constant symbols are the most common ones and are defined with the
+'config' statement. Non-constant symbols consist entirely of alphanumeric
+characters or underscores.
+Constant symbols are only part of expressions. Constant symbols are
+always surrounded by single or double quotes. Within the quote, any
+other character is allowed and the quotes can be escaped using '\'.
+
+Menu structure
+--------------
+
+The position of a menu entry in the tree is determined in two ways. First
+it can be specified explicitly::
+
+  menu "Network device support"
+	depends on NET
+
+  config NETDEVICES
+	...
+
+  endmenu
+
+All entries within the "menu" ... "endmenu" block become a submenu of
+"Network device support". All subentries inherit the dependencies from
+the menu entry, e.g. this means the dependency "NET" is added to the
+dependency list of the config option NETDEVICES.
+
+The other way to generate the menu structure is done by analyzing the
+dependencies. If a menu entry somehow depends on the previous entry, it
+can be made a submenu of it. First, the previous (parent) symbol must
+be part of the dependency list and then one of these two conditions
+must be true:
+
+- the child entry must become invisible, if the parent is set to 'n'
+- the child entry must only be visible, if the parent is visible::
+
+    config MODULES
+	bool "Enable loadable module support"
+
+    config MODVERSIONS
+	bool "Set version information on all module symbols"
+	depends on MODULES
+
+    comment "module support disabled"
+	depends on !MODULES
+
+MODVERSIONS directly depends on MODULES, this means it's only visible if
+MODULES is different from 'n'. The comment on the other hand is only
+visible when MODULES is set to 'n'.
+
+
+Kconfig syntax
+--------------
+
+The configuration file describes a series of menu entries, where every
+line starts with a keyword (except help texts). The following keywords
+end a menu entry:
+
+- config
+- menuconfig
+- choice/endchoice
+- comment
+- menu/endmenu
+- if/endif
+- source
+
+The first five also start the definition of a menu entry.
+
+config::
+
+	"config" <symbol>
+	<config options>
+
+This defines a config symbol <symbol> and accepts any of above
+attributes as options.
+
+menuconfig::
+
+	"menuconfig" <symbol>
+	<config options>
+
+This is similar to the simple config entry above, but it also gives a
+hint to front ends, that all suboptions should be displayed as a
+separate list of options. To make sure all the suboptions will really
+show up under the menuconfig entry and not outside of it, every item
+from the <config options> list must depend on the menuconfig symbol.
+In practice, this is achieved by using one of the next two constructs::
+
+  (1):
+  menuconfig M
+  if M
+      config C1
+      config C2
+  endif
+
+  (2):
+  menuconfig M
+  config C1
+      depends on M
+  config C2
+      depends on M
+
+In the following examples (3) and (4), C1 and C2 still have the M
+dependency, but will not appear under menuconfig M anymore, because
+of C0, which doesn't depend on M::
+
+  (3):
+  menuconfig M
+      config C0
+  if M
+      config C1
+      config C2
+  endif
+
+  (4):
+  menuconfig M
+  config C0
+  config C1
+      depends on M
+  config C2
+      depends on M
+
+choices::
+
+	"choice" [symbol]
+	<choice options>
+	<choice block>
+	"endchoice"
+
+This defines a choice group and accepts any of the above attributes as
+options. A choice can only be of type bool or tristate.  If no type is
+specified for a choice, its type will be determined by the type of
+the first choice element in the group or remain unknown if none of the
+choice elements have a type specified, as well.
+
+While a boolean choice only allows a single config entry to be
+selected, a tristate choice also allows any number of config entries
+to be set to 'm'. This can be used if multiple drivers for a single
+hardware exists and only a single driver can be compiled/loaded into
+the kernel, but all drivers can be compiled as modules.
+
+A choice accepts another option "optional", which allows to set the
+choice to 'n' and no entry needs to be selected.
+If no [symbol] is associated with a choice, then you can not have multiple
+definitions of that choice. If a [symbol] is associated to the choice,
+then you may define the same choice (i.e. with the same entries) in another
+place.
+
+comment::
+
+	"comment" <prompt>
+	<comment options>
+
+This defines a comment which is displayed to the user during the
+configuration process and is also echoed to the output files. The only
+possible options are dependencies.
+
+menu::
+
+	"menu" <prompt>
+	<menu options>
+	<menu block>
+	"endmenu"
+
+This defines a menu block, see "Menu structure" above for more
+information. The only possible options are dependencies and "visible"
+attributes.
+
+if::
+
+	"if" <expr>
+	<if block>
+	"endif"
+
+This defines an if block. The dependency expression <expr> is appended
+to all enclosed menu entries.
+
+source::
+
+	"source" <prompt>
+
+This reads the specified configuration file. This file is always parsed.
+
+mainmenu::
+
+	"mainmenu" <prompt>
+
+This sets the config program's title bar if the config program chooses
+to use it. It should be placed at the top of the configuration, before any
+other statement.
+
+'#' Kconfig source file comment:
+
+An unquoted '#' character anywhere in a source file line indicates
+the beginning of a source file comment.  The remainder of that line
+is a comment.
+
+
+Kconfig hints
+-------------
+This is a collection of Kconfig tips, most of which aren't obvious at
+first glance and most of which have become idioms in several Kconfig
+files.
+
+Adding common features and make the usage configurable
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It is a common idiom to implement a feature/functionality that are
+relevant for some architectures but not all.
+The recommended way to do so is to use a config variable named HAVE_*
+that is defined in a common Kconfig file and selected by the relevant
+architectures.
+An example is the generic IOMAP functionality.
+
+We would in lib/Kconfig see::
+
+  # Generic IOMAP is used to ...
+  config HAVE_GENERIC_IOMAP
+
+  config GENERIC_IOMAP
+	depends on HAVE_GENERIC_IOMAP && FOO
+
+And in lib/Makefile we would see::
+
+	obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
+
+For each architecture using the generic IOMAP functionality we would see::
+
+  config X86
+	select ...
+	select HAVE_GENERIC_IOMAP
+	select ...
+
+Note: we use the existing config option and avoid creating a new
+config variable to select HAVE_GENERIC_IOMAP.
+
+Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is
+introduced to overcome the limitation of select which will force a
+config option to 'y' no matter the dependencies.
+The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
+situation where select forces a symbol equals to 'y'.
+
+Adding features that need compiler support
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are several features that need compiler support. The recommended way
+to describe the dependency on the compiler feature is to use "depends on"
+followed by a test macro::
+
+  config STACKPROTECTOR
+	bool "Stack Protector buffer overflow detection"
+	depends on $(cc-option,-fstack-protector)
+	...
+
+If you need to expose a compiler capability to makefiles and/or C source files,
+`CC_HAS_` is the recommended prefix for the config option::
+
+  config CC_HAS_ASM_GOTO
+	def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
+
+Build as module only
+~~~~~~~~~~~~~~~~~~~~
+To restrict a component build to module-only, qualify its config symbol
+with "depends on m".  E.g.::
+
+  config FOO
+	depends on BAR && m
+
+limits FOO to module (=m) or disabled (=n).
+
+Kconfig recursive dependency limitations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you've hit the Kconfig error: "recursive dependency detected" you've run
+into a recursive dependency issue with Kconfig, a recursive dependency can be
+summarized as a circular dependency. The kconfig tools need to ensure that
+Kconfig files comply with specified configuration requirements. In order to do
+that kconfig must determine the values that are possible for all Kconfig
+symbols, this is currently not possible if there is a circular relation
+between two or more Kconfig symbols. For more details refer to the "Simple
+Kconfig recursive issue" subsection below. Kconfig does not do recursive
+dependency resolution; this has a few implications for Kconfig file writers.
+We'll first explain why this issues exists and then provide an example
+technical limitation which this brings upon Kconfig developers. Eager
+developers wishing to try to address this limitation should read the next
+subsections.
+
+Simple Kconfig recursive issue
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Read: Documentation/kbuild/Kconfig.recursion-issue-01
+
+Test with::
+
+  make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-01 allnoconfig
+
+Cumulative Kconfig recursive issue
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Read: Documentation/kbuild/Kconfig.recursion-issue-02
+
+Test with::
+
+  make KBUILD_KCONFIG=Documentation/kbuild/Kconfig.recursion-issue-02 allnoconfig
+
+Practical solutions to kconfig recursive issue
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Developers who run into the recursive Kconfig issue have two options
+at their disposal. We document them below and also provide a list of
+historical issues resolved through these different solutions.
+
+  a) Remove any superfluous "select FOO" or "depends on FOO"
+  b) Match dependency semantics:
+
+	b1) Swap all "select FOO" to "depends on FOO" or,
+
+	b2) Swap all "depends on FOO" to "select FOO"
+
+The resolution to a) can be tested with the sample Kconfig file
+Documentation/kbuild/Kconfig.recursion-issue-01 through the removal
+of the "select CORE" from CORE_BELL_A_ADVANCED as that is implicit already
+since CORE_BELL_A depends on CORE. At times it may not be possible to remove
+some dependency criteria, for such cases you can work with solution b).
+
+The two different resolutions for b) can be tested in the sample Kconfig file
+Documentation/kbuild/Kconfig.recursion-issue-02.
+
+Below is a list of examples of prior fixes for these types of recursive issues;
+all errors appear to involve one or more "select" statements and one or more
+"depends on".
+
+============    ===================================
+commit          fix
+============    ===================================
+06b718c01208    select A -> depends on A
+c22eacfe82f9    depends on A -> depends on B
+6a91e854442c    select A -> depends on A
+118c565a8f2e    select A -> select B
+f004e5594705    select A -> depends on A
+c7861f37b4c6    depends on A -> (null)
+80c69915e5fb    select A -> (null)              (1)
+c2218e26c0d0    select A -> depends on A        (1)
+d6ae99d04e1c    select A -> depends on A
+95ca19cf8cbf    select A -> depends on A
+8f057d7bca54    depends on A -> (null)
+8f057d7bca54    depends on A -> select A
+a0701f04846e    select A -> depends on A
+0c8b92f7f259    depends on A -> (null)
+e4e9e0540928    select A -> depends on A        (2)
+7453ea886e87    depends on A > (null)           (1)
+7b1fff7e4fdf    select A -> depends on A
+86c747d2a4f0    select A -> depends on A
+d9f9ab51e55e    select A -> depends on A
+0c51a4d8abd6    depends on A -> select A        (3)
+e98062ed6dc4    select A -> depends on A        (3)
+91e5d284a7f1    select A -> (null)
+============    ===================================
+
+(1) Partial (or no) quote of error.
+(2) That seems to be the gist of that fix.
+(3) Same error.
+
+Future kconfig work
+~~~~~~~~~~~~~~~~~~~
+
+Work on kconfig is welcomed on both areas of clarifying semantics and on
+evaluating the use of a full SAT solver for it. A full SAT solver can be
+desirable to enable more complex dependency mappings and / or queries,
+for instance on possible use case for a SAT solver could be that of handling
+the current known recursive dependency issues. It is not known if this would
+address such issues but such evaluation is desirable. If support for a full SAT
+solver proves too complex or that it cannot address recursive dependency issues
+Kconfig should have at least clear and well defined semantics which also
+addresses and documents limitations or requirements such as the ones dealing
+with recursive dependencies.
+
+Further work on both of these areas is welcomed on Kconfig. We elaborate
+on both of these in the next two subsections.
+
+Semantics of Kconfig
+~~~~~~~~~~~~~~~~~~~~
+
+The use of Kconfig is broad, Linux is now only one of Kconfig's users:
+one study has completed a broad analysis of Kconfig use in 12 projects [0]_.
+Despite its widespread use, and although this document does a reasonable job
+in documenting basic Kconfig syntax a more precise definition of Kconfig
+semantics is welcomed. One project deduced Kconfig semantics through
+the use of the xconfig configurator [1]_. Work should be done to confirm if
+the deduced semantics matches our intended Kconfig design goals.
+
+Having well defined semantics can be useful for tools for practical
+evaluation of dependencies, for instance one such case was work to
+express in boolean abstraction of the inferred semantics of Kconfig to
+translate Kconfig logic into boolean formulas and run a SAT solver on this to
+find dead code / features (always inactive), 114 dead features were found in
+Linux using this methodology [1]_ (Section 8: Threats to validity).
+
+Confirming this could prove useful as Kconfig stands as one of the leading
+industrial variability modeling languages [1]_ [2]_. Its study would help
+evaluate practical uses of such languages, their use was only theoretical
+and real world requirements were not well understood. As it stands though
+only reverse engineering techniques have been used to deduce semantics from
+variability modeling languages such as Kconfig [3]_.
+
+.. [0] https://www.eng.uwaterloo.ca/~shshe/kconfig_semantics.pdf
+.. [1] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
+.. [2] https://gsd.uwaterloo.ca/sites/default/files/ase241-berger_0.pdf
+.. [3] https://gsd.uwaterloo.ca/sites/default/files/icse2011.pdf
+
+Full SAT solver for Kconfig
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Although SAT solvers [4]_ haven't yet been used by Kconfig directly, as noted
+in the previous subsection, work has been done however to express in boolean
+abstraction the inferred semantics of Kconfig to translate Kconfig logic into
+boolean formulas and run a SAT solver on it [5]_. Another known related project
+is CADOS [6]_ (former VAMOS [7]_) and the tools, mainly undertaker [8]_, which
+has been introduced first with [9]_.  The basic concept of undertaker is to
+extract variability models from Kconfig and put them together with a
+propositional formula extracted from CPP #ifdefs and build-rules into a SAT
+solver in order to find dead code, dead files, and dead symbols. If using a SAT
+solver is desirable on Kconfig one approach would be to evaluate repurposing
+such efforts somehow on Kconfig. There is enough interest from mentors of
+existing projects to not only help advise how to integrate this work upstream
+but also help maintain it long term. Interested developers should visit:
+
+https://kernelnewbies.org/KernelProjects/kconfig-sat
+
+.. [4] https://www.cs.cornell.edu/~sabhar/chapters/SATSolvers-KR-Handbook.pdf
+.. [5] https://gsd.uwaterloo.ca/sites/default/files/vm-2013-berger.pdf
+.. [6] https://cados.cs.fau.de
+.. [7] https://vamos.cs.fau.de
+.. [8] https://undertaker.cs.fau.de
+.. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
diff --git a/Documentation/kbuild/kconfig-macro-language.rst b/Documentation/kbuild/kconfig-macro-language.rst
new file mode 100644
index 000000000..8b413ef96
--- /dev/null
+++ b/Documentation/kbuild/kconfig-macro-language.rst
@@ -0,0 +1,247 @@
+======================
+Kconfig macro language
+======================
+
+Concept
+-------
+
+The basic idea was inspired by Make. When we look at Make, we notice sort of
+two languages in one. One language describes dependency graphs consisting of
+targets and prerequisites. The other is a macro language for performing textual
+substitution.
+
+There is clear distinction between the two language stages. For example, you
+can write a makefile like follows::
+
+    APP := foo
+    SRC := foo.c
+    CC := gcc
+
+    $(APP): $(SRC)
+            $(CC) -o $(APP) $(SRC)
+
+The macro language replaces the variable references with their expanded form,
+and handles as if the source file were input like follows::
+
+    foo: foo.c
+            gcc -o foo foo.c
+
+Then, Make analyzes the dependency graph and determines the targets to be
+updated.
+
+The idea is quite similar in Kconfig - it is possible to describe a Kconfig
+file like this::
+
+    CC := gcc
+
+    config CC_HAS_FOO
+            def_bool $(shell, $(srctree)/scripts/gcc-check-foo.sh $(CC))
+
+The macro language in Kconfig processes the source file into the following
+intermediate::
+
+    config CC_HAS_FOO
+            def_bool y
+
+Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
+dependency as explained in kconfig-language.rst.
+
+
+Variables
+---------
+
+Like in Make, a variable in Kconfig works as a macro variable.  A macro
+variable is expanded "in place" to yield a text string that may then be
+expanded further. To get the value of a variable, enclose the variable name in
+$( ). The parentheses are required even for single-letter variable names; $X is
+a syntax error. The curly brace form as in ${CC} is not supported either.
+
+There are two types of variables: simply expanded variables and recursively
+expanded variables.
+
+A simply expanded variable is defined using the := assignment operator. Its
+righthand side is expanded immediately upon reading the line from the Kconfig
+file.
+
+A recursively expanded variable is defined using the = assignment operator.
+Its righthand side is simply stored as the value of the variable without
+expanding it in any way. Instead, the expansion is performed when the variable
+is used.
+
+There is another type of assignment operator; += is used to append text to a
+variable. The righthand side of += is expanded immediately if the lefthand
+side was originally defined as a simple variable. Otherwise, its evaluation is
+deferred.
+
+The variable reference can take parameters, in the following form::
+
+  $(name,arg1,arg2,arg3)
+
+You can consider the parameterized reference as a function. (more precisely,
+"user-defined function" in contrast to "built-in function" listed below).
+
+Useful functions must be expanded when they are used since the same function is
+expanded differently if different parameters are passed. Hence, a user-defined
+function is defined using the = assignment operator. The parameters are
+referenced within the body definition with $(1), $(2), etc.
+
+In fact, recursively expanded variables and user-defined functions are the same
+internally. (In other words, "variable" is "function with zero argument".)
+When we say "variable" in a broad sense, it includes "user-defined function".
+
+
+Built-in functions
+------------------
+
+Like Make, Kconfig provides several built-in functions. Every function takes a
+particular number of arguments.
+
+In Make, every built-in function takes at least one argument. Kconfig allows
+zero argument for built-in functions, such as $(fileno), $(lineno). You could
+consider those as "built-in variable", but it is just a matter of how we call
+it after all. Let's say "built-in function" here to refer to natively supported
+functionality.
+
+Kconfig currently supports the following built-in functions.
+
+ - $(shell,command)
+
+  The "shell" function accepts a single argument that is expanded and passed
+  to a subshell for execution. The standard output of the command is then read
+  and returned as the value of the function. Every newline in the output is
+  replaced with a space. Any trailing newlines are deleted. The standard error
+  is not returned, nor is any program exit status.
+
+ - $(info,text)
+
+  The "info" function takes a single argument and prints it to stdout.
+  It evaluates to an empty string.
+
+ - $(warning-if,condition,text)
+
+  The "warning-if" function takes two arguments. If the condition part is "y",
+  the text part is sent to stderr. The text is prefixed with the name of the
+  current Kconfig file and the current line number.
+
+ - $(error-if,condition,text)
+
+  The "error-if" function is similar to "warning-if", but it terminates the
+  parsing immediately if the condition part is "y".
+
+ - $(filename)
+
+  The 'filename' takes no argument, and $(filename) is expanded to the file
+  name being parsed.
+
+ - $(lineno)
+
+  The 'lineno' takes no argument, and $(lineno) is expanded to the line number
+  being parsed.
+
+
+Make vs Kconfig
+---------------
+
+Kconfig adopts Make-like macro language, but the function call syntax is
+slightly different.
+
+A function call in Make looks like this::
+
+  $(func-name arg1,arg2,arg3)
+
+The function name and the first argument are separated by at least one
+whitespace. Then, leading whitespaces are trimmed from the first argument,
+while whitespaces in the other arguments are kept. You need to use a kind of
+trick to start the first parameter with spaces. For example, if you want
+to make "info" function print "  hello", you can write like follows::
+
+  empty :=
+  space := $(empty) $(empty)
+  $(info $(space)$(space)hello)
+
+Kconfig uses only commas for delimiters, and keeps all whitespaces in the
+function call. Some people prefer putting a space after each comma delimiter::
+
+  $(func-name, arg1, arg2, arg3)
+
+In this case, "func-name" will receive " arg1", " arg2", " arg3". The presence
+of leading spaces may matter depending on the function. The same applies to
+Make - for example, $(subst .c, .o, $(sources)) is a typical mistake; it
+replaces ".c" with " .o".
+
+In Make, a user-defined function is referenced by using a built-in function,
+'call', like this::
+
+    $(call my-func,arg1,arg2,arg3)
+
+Kconfig invokes user-defined functions and built-in functions in the same way.
+The omission of 'call' makes the syntax shorter.
+
+In Make, some functions treat commas verbatim instead of argument separators.
+For example, $(shell echo hello, world) runs the command "echo hello, world".
+Likewise, $(info hello, world) prints "hello, world" to stdout. You could say
+this is _useful_ inconsistency.
+
+In Kconfig, for simpler implementation and grammatical consistency, commas that
+appear in the $( ) context are always delimiters. It means::
+
+  $(shell, echo hello, world)
+
+is an error because it is passing two parameters where the 'shell' function
+accepts only one. To pass commas in arguments, you can use the following trick::
+
+  comma := ,
+  $(shell, echo hello$(comma) world)
+
+
+Caveats
+-------
+
+A variable (or function) cannot be expanded across tokens. So, you cannot use
+a variable as a shorthand for an expression that consists of multiple tokens.
+The following works::
+
+    RANGE_MIN := 1
+    RANGE_MAX := 3
+
+    config FOO
+            int "foo"
+            range $(RANGE_MIN) $(RANGE_MAX)
+
+But, the following does not work::
+
+    RANGES := 1 3
+
+    config FOO
+            int "foo"
+            range $(RANGES)
+
+A variable cannot be expanded to any keyword in Kconfig.  The following does
+not work::
+
+    MY_TYPE := tristate
+
+    config FOO
+            $(MY_TYPE) "foo"
+            default y
+
+Obviously from the design, $(shell command) is expanded in the textual
+substitution phase. You cannot pass symbols to the 'shell' function.
+
+The following does not work as expected::
+
+    config ENDIAN_FLAG
+            string
+            default "-mbig-endian" if CPU_BIG_ENDIAN
+            default "-mlittle-endian" if CPU_LITTLE_ENDIAN
+
+    config CC_HAS_ENDIAN_FLAG
+            def_bool $(shell $(srctree)/scripts/gcc-check-flag ENDIAN_FLAG)
+
+Instead, you can do like follows so that any function call is statically
+expanded::
+
+    config CC_HAS_ENDIAN_FLAG
+            bool
+            default $(shell $(srctree)/scripts/gcc-check-flag -mbig-endian) if CPU_BIG_ENDIAN
+            default $(shell $(srctree)/scripts/gcc-check-flag -mlittle-endian) if CPU_LITTLE_ENDIAN
diff --git a/Documentation/kbuild/kconfig.rst b/Documentation/kbuild/kconfig.rst
new file mode 100644
index 000000000..dce6801d6
--- /dev/null
+++ b/Documentation/kbuild/kconfig.rst
@@ -0,0 +1,299 @@
+===================
+Kconfig make config
+===================
+
+This file contains some assistance for using `make *config`.
+
+Use "make help" to list all of the possible configuration targets.
+
+The xconfig ('qconf'), menuconfig ('mconf'), and nconfig ('nconf')
+programs also have embedded help text.  Be sure to check that for
+navigation, search, and other general help text.
+
+General
+-------
+
+New kernel releases often introduce new config symbols.  Often more
+important, new kernel releases may rename config symbols.  When
+this happens, using a previously working .config file and running
+"make oldconfig" won't necessarily produce a working new kernel
+for you, so you may find that you need to see what NEW kernel
+symbols have been introduced.
+
+To see a list of new config symbols, use::
+
+	cp user/some/old.config .config
+	make listnewconfig
+
+and the config program will list any new symbols, one per line.
+
+Alternatively, you can use the brute force method::
+
+	make oldconfig
+	scripts/diffconfig .config.old .config | less
+
+----------------------------------------------------------------------
+
+Environment variables for `*config`
+
+KCONFIG_CONFIG
+--------------
+This environment variable can be used to specify a default kernel config
+file name to override the default name of ".config".
+
+KCONFIG_OVERWRITECONFIG
+-----------------------
+If you set KCONFIG_OVERWRITECONFIG in the environment, Kconfig will not
+break symlinks when .config is a symlink to somewhere else.
+
+`CONFIG_`
+---------
+If you set `CONFIG_` in the environment, Kconfig will prefix all symbols
+with its value when saving the configuration, instead of using the default,
+`CONFIG_`.
+
+----------------------------------------------------------------------
+
+Environment variables for '{allyes/allmod/allno/rand}config'
+
+KCONFIG_ALLCONFIG
+-----------------
+(partially based on lkml email from/by Rob Landley, re: miniconfig)
+
+--------------------------------------------------
+
+The allyesconfig/allmodconfig/allnoconfig/randconfig variants can also
+use the environment variable KCONFIG_ALLCONFIG as a flag or a filename
+that contains config symbols that the user requires to be set to a
+specific value.  If KCONFIG_ALLCONFIG is used without a filename where
+KCONFIG_ALLCONFIG == "" or KCONFIG_ALLCONFIG == "1", `make *config`
+checks for a file named "all{yes/mod/no/def/random}.config"
+(corresponding to the `*config` command that was used) for symbol values
+that are to be forced.  If this file is not found, it checks for a
+file named "all.config" to contain forced values.
+
+This enables you to create "miniature" config (miniconfig) or custom
+config files containing just the config symbols that you are interested
+in.  Then the kernel config system generates the full .config file,
+including symbols of your miniconfig file.
+
+This 'KCONFIG_ALLCONFIG' file is a config file which contains
+(usually a subset of all) preset config symbols.  These variable
+settings are still subject to normal dependency checks.
+
+Examples::
+
+	KCONFIG_ALLCONFIG=custom-notebook.config make allnoconfig
+
+or::
+
+	KCONFIG_ALLCONFIG=mini.config make allnoconfig
+
+or::
+
+	make KCONFIG_ALLCONFIG=mini.config allnoconfig
+
+These examples will disable most options (allnoconfig) but enable or
+disable the options that are explicitly listed in the specified
+mini-config files.
+
+----------------------------------------------------------------------
+
+Environment variables for 'randconfig'
+
+KCONFIG_SEED
+------------
+You can set this to the integer value used to seed the RNG, if you want
+to somehow debug the behaviour of the kconfig parser/frontends.
+If not set, the current time will be used.
+
+KCONFIG_PROBABILITY
+-------------------
+This variable can be used to skew the probabilities. This variable can
+be unset or empty, or set to three different formats:
+
+    =======================     ==================  =====================
+	KCONFIG_PROBABILITY     y:n split           y:m:n split
+    =======================     ==================  =====================
+	unset or empty          50  : 50            33  : 33  : 34
+	N                        N  : 100-N         N/2 : N/2 : 100-N
+    [1] N:M                     N+M : 100-(N+M)      N  :  M  : 100-(N+M)
+    [2] N:M:L                    N  : 100-N          M  :  L  : 100-(M+L)
+    =======================     ==================  =====================
+
+where N, M and L are integers (in base 10) in the range [0,100], and so
+that:
+
+    [1] N+M is in the range [0,100]
+
+    [2] M+L is in the range [0,100]
+
+Examples::
+
+	KCONFIG_PROBABILITY=10
+		10% of booleans will be set to 'y', 90% to 'n'
+		5% of tristates will be set to 'y', 5% to 'm', 90% to 'n'
+	KCONFIG_PROBABILITY=15:25
+		40% of booleans will be set to 'y', 60% to 'n'
+		15% of tristates will be set to 'y', 25% to 'm', 60% to 'n'
+	KCONFIG_PROBABILITY=10:15:15
+		10% of booleans will be set to 'y', 90% to 'n'
+		15% of tristates will be set to 'y', 15% to 'm', 70% to 'n'
+
+----------------------------------------------------------------------
+
+Environment variables for 'syncconfig'
+
+KCONFIG_NOSILENTUPDATE
+----------------------
+If this variable has a non-blank value, it prevents silent kernel
+config updates (requires explicit updates).
+
+KCONFIG_AUTOCONFIG
+------------------
+This environment variable can be set to specify the path & name of the
+"auto.conf" file.  Its default value is "include/config/auto.conf".
+
+KCONFIG_AUTOHEADER
+------------------
+This environment variable can be set to specify the path & name of the
+"autoconf.h" (header) file.
+Its default value is "include/generated/autoconf.h".
+
+
+----------------------------------------------------------------------
+
+menuconfig
+----------
+
+SEARCHING for CONFIG symbols
+
+Searching in menuconfig:
+
+	The Search function searches for kernel configuration symbol
+	names, so you have to know something close to what you are
+	looking for.
+
+	Example::
+
+		/hotplug
+		This lists all config symbols that contain "hotplug",
+		e.g., HOTPLUG_CPU, MEMORY_HOTPLUG.
+
+	For search help, enter / followed by TAB-TAB (to highlight
+	<Help>) and Enter.  This will tell you that you can also use
+	regular expressions (regexes) in the search string, so if you
+	are not interested in MEMORY_HOTPLUG, you could try::
+
+		/^hotplug
+
+	When searching, symbols are sorted thus:
+
+	  - first, exact matches, sorted alphabetically (an exact match
+	    is when the search matches the complete symbol name);
+	  - then, other matches, sorted alphabetically.
+
+	For example: ^ATH.K matches:
+
+	    ATH5K ATH9K ATH5K_AHB ATH5K_DEBUG [...] ATH6KL ATH6KL_DEBUG
+	    [...] ATH9K_AHB ATH9K_BTCOEX_SUPPORT ATH9K_COMMON [...]
+
+	of which only ATH5K and ATH9K match exactly and so are sorted
+	first (and in alphabetical order), then come all other symbols,
+	sorted in alphabetical order.
+
+----------------------------------------------------------------------
+
+User interface options for 'menuconfig'
+
+MENUCONFIG_COLOR
+----------------
+It is possible to select different color themes using the variable
+MENUCONFIG_COLOR.  To select a theme use::
+
+	make MENUCONFIG_COLOR=<theme> menuconfig
+
+Available themes are::
+
+  - mono       => selects colors suitable for monochrome displays
+  - blackbg    => selects a color scheme with black background
+  - classic    => theme with blue background. The classic look
+  - bluetitle  => a LCD friendly version of classic. (default)
+
+MENUCONFIG_MODE
+---------------
+This mode shows all sub-menus in one large tree.
+
+Example::
+
+	make MENUCONFIG_MODE=single_menu menuconfig
+
+----------------------------------------------------------------------
+
+nconfig
+-------
+
+nconfig is an alternate text-based configurator.  It lists function
+keys across the bottom of the terminal (window) that execute commands.
+You can also just use the corresponding numeric key to execute the
+commands unless you are in a data entry window.  E.g., instead of F6
+for Save, you can just press 6.
+
+Use F1 for Global help or F3 for the Short help menu.
+
+Searching in nconfig:
+
+	You can search either in the menu entry "prompt" strings
+	or in the configuration symbols.
+
+	Use / to begin a search through the menu entries.  This does
+	not support regular expressions.  Use <Down> or <Up> for
+	Next hit and Previous hit, respectively.  Use <Esc> to
+	terminate the search mode.
+
+	F8 (SymSearch) searches the configuration symbols for the
+	given string or regular expression (regex).
+
+NCONFIG_MODE
+------------
+This mode shows all sub-menus in one large tree.
+
+Example::
+
+	make NCONFIG_MODE=single_menu nconfig
+
+----------------------------------------------------------------------
+
+xconfig
+-------
+
+Searching in xconfig:
+
+	The Search function searches for kernel configuration symbol
+	names, so you have to know something close to what you are
+	looking for.
+
+	Example::
+
+		Ctrl-F hotplug
+
+	or::
+
+		Menu: File, Search, hotplug
+
+	lists all config symbol entries that contain "hotplug" in
+	the symbol name.  In this Search dialog, you may change the
+	config setting for any of the entries that are not grayed out.
+	You can also enter a different search string without having
+	to return to the main menu.
+
+
+----------------------------------------------------------------------
+
+gconfig
+-------
+
+Searching in gconfig:
+
+	There is no search command in gconfig.  However, gconfig does
+	have several different viewing choices, modes, and options.
diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
new file mode 100644
index 000000000..21c847890
--- /dev/null
+++ b/Documentation/kbuild/llvm.rst
@@ -0,0 +1,90 @@
+.. _kbuild_llvm:
+
+==============================
+Building Linux with Clang/LLVM
+==============================
+
+This document covers how to build the Linux kernel with Clang and LLVM
+utilities.
+
+About
+-----
+
+The Linux kernel has always traditionally been compiled with GNU toolchains
+such as GCC and binutils. Ongoing work has allowed for `Clang
+<https://clang.llvm.org/>`_ and `LLVM <https://llvm.org/>`_ utilities to be
+used as viable substitutes. Distributions such as `Android
+<https://www.android.com/>`_, `ChromeOS
+<https://www.chromium.org/chromium-os>`_, and `OpenMandriva
+<https://www.openmandriva.org/>`_ use Clang built kernels.  `LLVM is a
+collection of toolchain components implemented in terms of C++ objects
+<https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM that
+supports C and the GNU C extensions required by the kernel, and is pronounced
+"klang," not "see-lang."
+
+Clang
+-----
+
+The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
+``CC=`` should be set when selecting a config and during a build. ::
+
+	make CC=clang defconfig
+
+	make CC=clang
+
+Cross Compiling
+---------------
+
+A single Clang compiler binary will typically contain all supported backends,
+which can help simplify cross compiling. ::
+
+	ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
+
+``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
+``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
+example: ::
+
+	clang --target=aarch64-linux-gnu foo.c
+
+LLVM Utilities
+--------------
+
+LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
+to enable them. ::
+
+	make LLVM=1
+
+They can be enabled individually. The full list of the parameters: ::
+
+	make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
+	  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
+	  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
+
+Currently, the integrated assembler is disabled by default. You can pass
+``LLVM_IAS=1`` to enable it.
+
+Getting Help
+------------
+
+- `Website <https://clangbuiltlinux.github.io/>`_
+- `Mailing List <https://groups.google.com/forum/#!forum/clang-built-linux>`_: <clang-built-linux@googlegroups.com>
+- `Issue Tracker <https://github.com/ClangBuiltLinux/linux/issues>`_
+- IRC: #clangbuiltlinux on chat.freenode.net
+- `Telegram <https://t.me/ClangBuiltLinux>`_: @ClangBuiltLinux
+- `Wiki <https://github.com/ClangBuiltLinux/linux/wiki>`_
+- `Beginner Bugs <https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22>`_
+
+.. _getting_llvm:
+
+Getting LLVM
+-------------
+
+- https://releases.llvm.org/download.html
+- https://github.com/llvm/llvm-project
+- https://llvm.org/docs/GettingStarted.html
+- https://llvm.org/docs/CMake.html
+- https://apt.llvm.org/
+- https://www.archlinux.org/packages/extra/x86_64/llvm/
+- https://github.com/ClangBuiltLinux/tc-build
+- https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source
+- https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/
diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
new file mode 100644
index 000000000..0d5dd5413
--- /dev/null
+++ b/Documentation/kbuild/makefiles.rst
@@ -0,0 +1,1624 @@
+======================
+Linux Kernel Makefiles
+======================
+
+This document describes the Linux kernel Makefiles.
+
+.. Table of Contents
+
+	=== 1 Overview
+	=== 2 Who does what
+	=== 3 The kbuild files
+	   --- 3.1 Goal definitions
+	   --- 3.2 Built-in object goals - obj-y
+	   --- 3.3 Loadable module goals - obj-m
+	   --- 3.4 Objects which export symbols
+	   --- 3.5 Library file goals - lib-y
+	   --- 3.6 Descending down in directories
+	   --- 3.7 Compilation flags
+	   --- 3.8 <deleted>
+	   --- 3.9 Dependency tracking
+	   --- 3.10 Special Rules
+	   --- 3.11 $(CC) support functions
+	   --- 3.12 $(LD) support functions
+	   --- 3.13 Script Invocation
+
+	=== 4 Host Program support
+	   --- 4.1 Simple Host Program
+	   --- 4.2 Composite Host Programs
+	   --- 4.3 Using C++ for host programs
+	   --- 4.4 Controlling compiler options for host programs
+	   --- 4.5 When host programs are actually built
+
+	=== 5 Userspace Program support
+	   --- 5.1 Simple Userspace Program
+	   --- 5.2 Composite Userspace Programs
+	   --- 5.3 Controlling compiler options for userspace programs
+	   --- 5.4 When userspace programs are actually built
+
+	=== 6 Kbuild clean infrastructure
+
+	=== 7 Architecture Makefiles
+	   --- 7.1 Set variables to tweak the build to the architecture
+	   --- 7.2 Add prerequisites to archheaders
+	   --- 7.3 Add prerequisites to archprepare
+	   --- 7.4 List directories to visit when descending
+	   --- 7.5 Architecture-specific boot images
+	   --- 7.6 Building non-kbuild targets
+	   --- 7.7 Commands useful for building a boot image
+	   --- 7.8 Custom kbuild commands
+	   --- 7.9 Preprocessing linker scripts
+	   --- 7.10 Generic header files
+	   --- 7.11 Post-link pass
+
+	=== 8 Kbuild syntax for exported headers
+		--- 8.1 no-export-headers
+		--- 8.2 generic-y
+		--- 8.3 generated-y
+		--- 8.4 mandatory-y
+
+	=== 9 Kbuild Variables
+	=== 10 Makefile language
+	=== 11 Credits
+	=== 12 TODO
+
+1 Overview
+==========
+
+The Makefiles have five parts::
+
+	Makefile		the top Makefile.
+	.config			the kernel configuration file.
+	arch/$(ARCH)/Makefile	the arch Makefile.
+	scripts/Makefile.*	common rules etc. for all kbuild Makefiles.
+	kbuild Makefiles	there are about 500 of these.
+
+The top Makefile reads the .config file, which comes from the kernel
+configuration process.
+
+The top Makefile is responsible for building two major products: vmlinux
+(the resident kernel image) and modules (any module files).
+It builds these goals by recursively descending into the subdirectories of
+the kernel source tree.
+The list of subdirectories which are visited depends upon the kernel
+configuration. The top Makefile textually includes an arch Makefile
+with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
+architecture-specific information to the top Makefile.
+
+Each subdirectory has a kbuild Makefile which carries out the commands
+passed down from above. The kbuild Makefile uses information from the
+.config file to construct various file lists used by kbuild to build
+any built-in or modular targets.
+
+scripts/Makefile.* contains all the definitions/rules etc. that
+are used to build the kernel based on the kbuild makefiles.
+
+
+2 Who does what
+===============
+
+People have four different relationships with the kernel Makefiles.
+
+*Users* are people who build kernels.  These people type commands such as
+"make menuconfig" or "make".  They usually do not read or edit
+any kernel Makefiles (or any other source files).
+
+*Normal developers* are people who work on features such as device
+drivers, file systems, and network protocols.  These people need to
+maintain the kbuild Makefiles for the subsystem they are
+working on.  In order to do this effectively, they need some overall
+knowledge about the kernel Makefiles, plus detailed knowledge about the
+public interface for kbuild.
+
+*Arch developers* are people who work on an entire architecture, such
+as sparc or ia64.  Arch developers need to know about the arch Makefile
+as well as kbuild Makefiles.
+
+*Kbuild developers* are people who work on the kernel build system itself.
+These people need to know about all aspects of the kernel Makefiles.
+
+This document is aimed towards normal developers and arch developers.
+
+
+3 The kbuild files
+==================
+
+Most Makefiles within the kernel are kbuild Makefiles that use the
+kbuild infrastructure. This chapter introduces the syntax used in the
+kbuild makefiles.
+The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
+be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
+file will be used.
+
+Section 3.1 "Goal definitions" is a quick intro; further chapters provide
+more details, with real examples.
+
+3.1 Goal definitions
+--------------------
+
+	Goal definitions are the main part (heart) of the kbuild Makefile.
+	These lines define the files to be built, any special compilation
+	options, and any subdirectories to be entered recursively.
+
+	The most simple kbuild makefile contains one line:
+
+	Example::
+
+		obj-y += foo.o
+
+	This tells kbuild that there is one object in that directory, named
+	foo.o. foo.o will be built from foo.c or foo.S.
+
+	If foo.o shall be built as a module, the variable obj-m is used.
+	Therefore the following pattern is often used:
+
+	Example::
+
+		obj-$(CONFIG_FOO) += foo.o
+
+	$(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
+	If CONFIG_FOO is neither y nor m, then the file will not be compiled
+	nor linked.
+
+3.2 Built-in object goals - obj-y
+---------------------------------
+
+	The kbuild Makefile specifies object files for vmlinux
+	in the $(obj-y) lists.  These lists depend on the kernel
+	configuration.
+
+	Kbuild compiles all the $(obj-y) files.  It then calls
+	"$(AR) rcSTP" to merge these files into one built-in.a file.
+	This is a thin archive without a symbol table. It will be later
+	linked into vmlinux by scripts/link-vmlinux.sh
+
+	The order of files in $(obj-y) is significant.  Duplicates in
+	the lists are allowed: the first instance will be linked into
+	built-in.a and succeeding instances will be ignored.
+
+	Link order is significant, because certain functions
+	(module_init() / __initcall) will be called during boot in the
+	order they appear. So keep in mind that changing the link
+	order may e.g. change the order in which your SCSI
+	controllers are detected, and thus your disks are renumbered.
+
+	Example::
+
+		#drivers/isdn/i4l/Makefile
+		# Makefile for the kernel ISDN subsystem and device drivers.
+		# Each configuration option enables a list of files.
+		obj-$(CONFIG_ISDN_I4L)         += isdn.o
+		obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+3.3 Loadable module goals - obj-m
+---------------------------------
+
+	$(obj-m) specifies object files which are built as loadable
+	kernel modules.
+
+	A module may be built from one source file or several source
+	files. In the case of one source file, the kbuild makefile
+	simply adds the file to $(obj-m).
+
+	Example::
+
+		#drivers/isdn/i4l/Makefile
+		obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+	Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
+
+	If a kernel module is built from several source files, you specify
+	that you want to build a module in the same way as above; however,
+	kbuild needs to know which object files you want to build your
+	module from, so you have to tell it by setting a $(<module_name>-y)
+	variable.
+
+	Example::
+
+		#drivers/isdn/i4l/Makefile
+		obj-$(CONFIG_ISDN_I4L) += isdn.o
+		isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
+
+	In this example, the module name will be isdn.o. Kbuild will
+	compile the objects listed in $(isdn-y) and then run
+	"$(LD) -r" on the list of these files to generate isdn.o.
+
+	Due to kbuild recognizing $(<module_name>-y) for composite objects,
+	you can use the value of a `CONFIG_` symbol to optionally include an
+	object file as part of a composite object.
+
+	Example::
+
+		#fs/ext2/Makefile
+	        obj-$(CONFIG_EXT2_FS) += ext2.o
+		ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
+			  namei.o super.o symlink.o
+	        ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
+						xattr_trusted.o
+
+	In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
+	part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
+	evaluates to 'y'.
+
+	Note: Of course, when you are building objects into the kernel,
+	the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
+	kbuild will build an ext2.o file for you out of the individual
+	parts and then link this into built-in.a, as you would expect.
+
+3.4 Objects which export symbols
+--------------------------------
+
+	No special notation is required in the makefiles for
+	modules exporting symbols.
+
+3.5 Library file goals - lib-y
+------------------------------
+
+	Objects listed with obj-* are used for modules, or
+	combined in a built-in.a for that specific directory.
+	There is also the possibility to list objects that will
+	be included in a library, lib.a.
+	All objects listed with lib-y are combined in a single
+	library for that directory.
+	Objects that are listed in obj-y and additionally listed in
+	lib-y will not be included in the library, since they will
+	be accessible anyway.
+	For consistency, objects listed in lib-m will be included in lib.a.
+
+	Note that the same kbuild makefile may list files to be built-in
+	and to be part of a library. Therefore the same directory
+	may contain both a built-in.a and a lib.a file.
+
+	Example::
+
+		#arch/x86/lib/Makefile
+		lib-y    := delay.o
+
+	This will create a library lib.a based on delay.o. For kbuild to
+	actually recognize that there is a lib.a being built, the directory
+	shall be listed in libs-y.
+
+	See also "6.4 List directories to visit when descending".
+
+	Use of lib-y is normally restricted to `lib/` and `arch/*/lib`.
+
+3.6 Descending down in directories
+----------------------------------
+
+	A Makefile is only responsible for building objects in its own
+	directory. Files in subdirectories should be taken care of by
+	Makefiles in these subdirs. The build system will automatically
+	invoke make recursively in subdirectories, provided you let it know of
+	them.
+
+	To do so, obj-y and obj-m are used.
+	ext2 lives in a separate directory, and the Makefile present in fs/
+	tells kbuild to descend down using the following assignment.
+
+	Example::
+
+		#fs/Makefile
+		obj-$(CONFIG_EXT2_FS) += ext2/
+
+	If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
+	the corresponding obj- variable will be set, and kbuild will descend
+	down in the ext2 directory.
+
+	Kbuild uses this information not only to decide that it needs to visit
+	the directory, but also to decide whether or not to link objects from
+	the directory into vmlinux.
+
+	When Kbuild descends into the directory with 'y', all built-in objects
+	from that directory are combined into the built-in.a, which will be
+	eventually linked into vmlinux.
+
+	When Kbuild descends into the directory with 'm', in contrast, nothing
+	from that directory will be linked into vmlinux. If the Makefile in
+	that directory specifies obj-y, those objects will be left orphan.
+	It is very likely a bug of the Makefile or of dependencies in Kconfig.
+
+	It is good practice to use a `CONFIG_` variable when assigning directory
+	names. This allows kbuild to totally skip the directory if the
+	corresponding `CONFIG_` option is neither 'y' nor 'm'.
+
+3.7 Compilation flags
+---------------------
+
+    ccflags-y, asflags-y and ldflags-y
+	These three flags apply only to the kbuild makefile in which they
+	are assigned. They are used for all the normal cc, as and ld
+	invocations happening during a recursive build.
+	Note: Flags with the same behaviour were previously named:
+	EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
+	They are still supported but their usage is deprecated.
+
+	ccflags-y specifies options for compiling with $(CC).
+
+	Example::
+
+		# drivers/acpi/acpica/Makefile
+		ccflags-y			:= -Os -D_LINUX -DBUILDING_ACPICA
+		ccflags-$(CONFIG_ACPI_DEBUG)	+= -DACPI_DEBUG_OUTPUT
+
+	This variable is necessary because the top Makefile owns the
+	variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
+	entire tree.
+
+	asflags-y specifies assembler options.
+
+	Example::
+
+		#arch/sparc/kernel/Makefile
+		asflags-y := -ansi
+
+	ldflags-y specifies options for linking with $(LD).
+
+	Example::
+
+		#arch/cris/boot/compressed/Makefile
+		ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
+
+    subdir-ccflags-y, subdir-asflags-y
+	The two flags listed above are similar to ccflags-y and asflags-y.
+	The difference is that the subdir- variants have effect for the kbuild
+	file where they are present and all subdirectories.
+	Options specified using subdir-* are added to the commandline before
+	the options specified using the non-subdir variants.
+
+	Example::
+
+		subdir-ccflags-y := -Werror
+
+    ccflags-remove-y, asflags-remove-y
+	These flags are used to remove particular flags for the compiler,
+	assembler invocations.
+
+	Example::
+
+		ccflags-remove-$(CONFIG_MCOUNT) += -pg
+
+    CFLAGS_$@, AFLAGS_$@
+	CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
+	kbuild makefile.
+
+	$(CFLAGS_$@) specifies per-file options for $(CC).  The $@
+	part has a literal value which specifies the file that it is for.
+
+	CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
+	can re-add compiler flags that were removed by ccflags-remove-y.
+
+	Example::
+
+		# drivers/scsi/Makefile
+		CFLAGS_aha152x.o =   -DAHA152X_STAT -DAUTOCONF
+		CFLAGS_gdth.o    = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
+				     -DGDTH_STATISTICS
+
+	These two lines specify compilation flags for aha152x.o and gdth.o.
+
+	$(AFLAGS_$@) is a similar feature for source files in assembly
+	languages.
+
+	AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
+	can re-add assembler flags that were removed by asflags-remove-y.
+
+	Example::
+
+		# arch/arm/kernel/Makefile
+		AFLAGS_head.o        := -DTEXT_OFFSET=$(TEXT_OFFSET)
+		AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
+		AFLAGS_iwmmxt.o      := -Wa,-mcpu=iwmmxt
+
+
+3.9 Dependency tracking
+-----------------------
+
+	Kbuild tracks dependencies on the following:
+
+	1) All prerequisite files (both `*.c` and `*.h`)
+	2) `CONFIG_` options used in all prerequisite files
+	3) Command-line used to compile target
+
+	Thus, if you change an option to $(CC) all affected files will
+	be re-compiled.
+
+3.10 Special Rules
+------------------
+
+	Special rules are used when the kbuild infrastructure does
+	not provide the required support. A typical example is
+	header files generated during the build process.
+	Another example are the architecture-specific Makefiles which
+	need special rules to prepare boot images etc.
+
+	Special rules are written as normal Make rules.
+	Kbuild is not executing in the directory where the Makefile is
+	located, so all special rules shall provide a relative
+	path to prerequisite files and target files.
+
+	Two variables are used when defining special rules:
+
+	$(src)
+	    $(src) is a relative path which points to the directory
+	    where the Makefile is located. Always use $(src) when
+	    referring to files located in the src tree.
+
+	$(obj)
+	    $(obj) is a relative path which points to the directory
+	    where the target is saved. Always use $(obj) when
+	    referring to generated files.
+
+	    Example::
+
+		#drivers/scsi/Makefile
+		$(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
+			$(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
+
+	    This is a special rule, following the normal syntax
+	    required by make.
+
+	    The target file depends on two prerequisite files. References
+	    to the target file are prefixed with $(obj), references
+	    to prerequisites are referenced with $(src) (because they are not
+	    generated files).
+
+	$(kecho)
+	    echoing information to user in a rule is often a good practice
+	    but when execution "make -s" one does not expect to see any output
+	    except for warnings/errors.
+	    To support this kbuild defines $(kecho) which will echo out the
+	    text following $(kecho) to stdout except if "make -s" is used.
+
+	Example::
+
+		#arch/blackfin/boot/Makefile
+		$(obj)/vmImage: $(obj)/vmlinux.gz
+			$(call if_changed,uimage)
+			@$(kecho) 'Kernel: $@ is ready'
+
+
+3.11 $(CC) support functions
+----------------------------
+
+	The kernel may be built with several different versions of
+	$(CC), each supporting a unique set of features and options.
+	kbuild provides basic support to check for valid options for $(CC).
+	$(CC) is usually the gcc compiler, but other alternatives are
+	available.
+
+    as-option
+	as-option is used to check if $(CC) -- when used to compile
+	assembler (`*.S`) files -- supports the given option. An optional
+	second option may be specified if the first option is not supported.
+
+	Example::
+
+		#arch/sh/Makefile
+		cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
+
+	In the above example, cflags-y will be assigned the option
+	-Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
+	The second argument is optional, and if supplied will be used
+	if first argument is not supported.
+
+    as-instr
+	as-instr checks if the assembler reports a specific instruction
+	and then outputs either option1 or option2
+	C escapes are supported in the test instruction
+	Note: as-instr-option uses KBUILD_AFLAGS for assembler options
+
+    cc-option
+	cc-option is used to check if $(CC) supports a given option, and if
+	not supported to use an optional second option.
+
+	Example::
+
+		#arch/x86/Makefile
+		cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
+
+	In the above example, cflags-y will be assigned the option
+	-march=pentium-mmx if supported by $(CC), otherwise -march=i586.
+	The second argument to cc-option is optional, and if omitted,
+	cflags-y will be assigned no value if first option is not supported.
+	Note: cc-option uses KBUILD_CFLAGS for $(CC) options
+
+   cc-option-yn
+	cc-option-yn is used to check if gcc supports a given option
+	and return 'y' if supported, otherwise 'n'.
+
+	Example::
+
+		#arch/ppc/Makefile
+		biarch := $(call cc-option-yn, -m32)
+		aflags-$(biarch) += -a32
+		cflags-$(biarch) += -m32
+
+	In the above example, $(biarch) is set to y if $(CC) supports the -m32
+	option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
+	and $(cflags-y) will be assigned the values -a32 and -m32,
+	respectively.
+	Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
+
+    cc-disable-warning
+	cc-disable-warning checks if gcc supports a given warning and returns
+	the commandline switch to disable it. This special function is needed,
+	because gcc 4.4 and later accept any unknown -Wno-* option and only
+	warn about it if there is another warning in the source file.
+
+	Example::
+
+		KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
+
+	In the above example, -Wno-unused-but-set-variable will be added to
+	KBUILD_CFLAGS only if gcc really accepts it.
+
+    cc-ifversion
+	cc-ifversion tests the version of $(CC) and equals the fourth parameter
+	if version expression is true, or the fifth (if given) if the version
+	expression is false.
+
+	Example::
+
+		#fs/reiserfs/Makefile
+		ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
+
+	In this example, ccflags-y will be assigned the value -O1 if the
+	$(CC) version is less than 4.2.
+	cc-ifversion takes all the shell operators:
+	-eq, -ne, -lt, -le, -gt, and -ge
+	The third parameter may be a text as in this example, but it may also
+	be an expanded variable or a macro.
+
+    cc-cross-prefix
+	cc-cross-prefix is used to check if there exists a $(CC) in path with
+	one of the listed prefixes. The first prefix where there exist a
+	prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
+	then nothing is returned.
+	Additional prefixes are separated by a single space in the
+	call of cc-cross-prefix.
+	This functionality is useful for architecture Makefiles that try
+	to set CROSS_COMPILE to well-known values but may have several
+	values to select between.
+	It is recommended only to try to set CROSS_COMPILE if it is a cross
+	build (host arch is different from target arch). And if CROSS_COMPILE
+	is already set then leave it with the old value.
+
+	Example::
+
+		#arch/m68k/Makefile
+		ifneq ($(SUBARCH),$(ARCH))
+		        ifeq ($(CROSS_COMPILE),)
+		               CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
+			endif
+		endif
+
+3.12 $(LD) support functions
+----------------------------
+
+    ld-option
+	ld-option is used to check if $(LD) supports the supplied option.
+	ld-option takes two options as arguments.
+	The second argument is an optional option that can be used if the
+	first option is not supported by $(LD).
+
+	Example::
+
+		#Makefile
+		LDFLAGS_vmlinux += $(call ld-option, -X)
+
+3.13 Script invocation
+----------------------
+
+	Make rules may invoke scripts to build the kernel. The rules shall
+	always provide the appropriate interpreter to execute the script. They
+	shall not rely on the execute bits being set, and shall not invoke the
+	script directly. For the convenience of manual script invocation, such
+	as invoking ./scripts/checkpatch.pl, it is recommended to set execute
+	bits on the scripts nonetheless.
+
+	Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
+	$(PYTHON) and $(PYTHON3) to refer to interpreters for the respective
+	scripts.
+
+	Example::
+
+		#Makefile
+		cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
+			     $(KERNELRELEASE)
+
+4 Host Program support
+======================
+
+Kbuild supports building executables on the host for use during the
+compilation stage.
+Two steps are required in order to use a host executable.
+
+The first step is to tell kbuild that a host program exists. This is
+done utilising the variable "hostprogs".
+
+The second step is to add an explicit dependency to the executable.
+This can be done in two ways. Either add the dependency in a rule,
+or utilise the variable "always-y".
+Both possibilities are described in the following.
+
+4.1 Simple Host Program
+-----------------------
+
+	In some cases there is a need to compile and run a program on the
+	computer where the build is running.
+	The following line tells kbuild that the program bin2hex shall be
+	built on the build host.
+
+	Example::
+
+		hostprogs := bin2hex
+
+	Kbuild assumes in the above example that bin2hex is made from a single
+	c-source file named bin2hex.c located in the same directory as
+	the Makefile.
+
+4.2 Composite Host Programs
+---------------------------
+
+	Host programs can be made up based on composite objects.
+	The syntax used to define composite objects for host programs is
+	similar to the syntax used for kernel objects.
+	$(<executable>-objs) lists all objects used to link the final
+	executable.
+
+	Example::
+
+		#scripts/lxdialog/Makefile
+		hostprogs     := lxdialog
+		lxdialog-objs := checklist.o lxdialog.o
+
+	Objects with extension .o are compiled from the corresponding .c
+	files. In the above example, checklist.c is compiled to checklist.o
+	and lxdialog.c is compiled to lxdialog.o.
+
+	Finally, the two .o files are linked to the executable, lxdialog.
+	Note: The syntax <executable>-y is not permitted for host-programs.
+
+4.3 Using C++ for host programs
+-------------------------------
+
+	kbuild offers support for host programs written in C++. This was
+	introduced solely to support kconfig, and is not recommended
+	for general use.
+
+	Example::
+
+		#scripts/kconfig/Makefile
+		hostprogs     := qconf
+		qconf-cxxobjs := qconf.o
+
+	In the example above the executable is composed of the C++ file
+	qconf.cc - identified by $(qconf-cxxobjs).
+
+	If qconf is composed of a mixture of .c and .cc files, then an
+	additional line can be used to identify this.
+
+	Example::
+
+		#scripts/kconfig/Makefile
+		hostprogs     := qconf
+		qconf-cxxobjs := qconf.o
+		qconf-objs    := check.o
+
+4.4 Controlling compiler options for host programs
+--------------------------------------------------
+
+	When compiling host programs, it is possible to set specific flags.
+	The programs will always be compiled utilising $(HOSTCC) passed
+	the options specified in $(KBUILD_HOSTCFLAGS).
+	To set flags that will take effect for all host programs created
+	in that Makefile, use the variable HOST_EXTRACFLAGS.
+
+	Example::
+
+		#scripts/lxdialog/Makefile
+		HOST_EXTRACFLAGS += -I/usr/include/ncurses
+
+	To set specific flags for a single file the following construction
+	is used:
+
+	Example::
+
+		#arch/ppc64/boot/Makefile
+		HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
+
+	It is also possible to specify additional options to the linker.
+
+	Example::
+
+		#scripts/kconfig/Makefile
+		HOSTLDLIBS_qconf := -L$(QTDIR)/lib
+
+	When linking qconf, it will be passed the extra option
+	"-L$(QTDIR)/lib".
+
+4.5 When host programs are actually built
+-----------------------------------------
+
+	Kbuild will only build host-programs when they are referenced
+	as a prerequisite.
+	This is possible in two ways:
+
+	(1) List the prerequisite explicitly in a special rule.
+
+	Example::
+
+		#drivers/pci/Makefile
+		hostprogs := gen-devlist
+		$(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
+			( cd $(obj); ./gen-devlist ) < $<
+
+	The target $(obj)/devlist.h will not be built before
+	$(obj)/gen-devlist is updated. Note that references to
+	the host programs in special rules must be prefixed with $(obj).
+
+	(2) Use always-y
+
+	When there is no suitable special rule, and the host program
+	shall be built when a makefile is entered, the always-y
+	variable shall be used.
+
+	Example::
+
+		#scripts/lxdialog/Makefile
+		hostprogs     := lxdialog
+		always-y      := $(hostprogs)
+
+	Kbuild provides the following shorthand for this:
+
+		hostprogs-always-y := lxdialog
+
+	This will tell kbuild to build lxdialog even if not referenced in
+	any rule.
+
+5 Userspace Program support
+===========================
+
+Just like host programs, Kbuild also supports building userspace executables
+for the target architecture (i.e. the same architecture as you are building
+the kernel for).
+
+The syntax is quite similar. The difference is to use "userprogs" instead of
+"hostprogs".
+
+5.1 Simple Userspace Program
+----------------------------
+
+	The following line tells kbuild that the program bpf-direct shall be
+	built for the target architecture.
+
+	Example::
+
+		userprogs := bpf-direct
+
+	Kbuild assumes in the above example that bpf-direct is made from a
+	single C source file named bpf-direct.c located in the same directory
+	as the Makefile.
+
+5.2 Composite Userspace Programs
+--------------------------------
+
+	Userspace programs can be made up based on composite objects.
+	The syntax used to define composite objects for userspace programs is
+	similar to the syntax used for kernel objects.
+	$(<executable>-objs) lists all objects used to link the final
+	executable.
+
+	Example::
+
+		#samples/seccomp/Makefile
+		userprogs      := bpf-fancy
+		bpf-fancy-objs := bpf-fancy.o bpf-helper.o
+
+	Objects with extension .o are compiled from the corresponding .c
+	files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
+	and bpf-helper.c is compiled to bpf-helper.o.
+
+	Finally, the two .o files are linked to the executable, bpf-fancy.
+	Note: The syntax <executable>-y is not permitted for userspace programs.
+
+5.3 Controlling compiler options for userspace programs
+-------------------------------------------------------
+
+	When compiling userspace programs, it is possible to set specific flags.
+	The programs will always be compiled utilising $(CC) passed
+	the options specified in $(KBUILD_USERCFLAGS).
+	To set flags that will take effect for all userspace programs created
+	in that Makefile, use the variable userccflags.
+
+	Example::
+
+		# samples/seccomp/Makefile
+		userccflags += -I usr/include
+
+	To set specific flags for a single file the following construction
+	is used:
+
+	Example::
+
+		bpf-helper-userccflags += -I user/include
+
+	It is also possible to specify additional options to the linker.
+
+	Example::
+
+		# net/bpfilter/Makefile
+		bpfilter_umh-userldflags += -static
+
+	When linking bpfilter_umh, it will be passed the extra option -static.
+
+5.4 When userspace programs are actually built
+----------------------------------------------
+
+	Kbuild builds userspace programs only when told to do so.
+	There are two ways to do this.
+
+	(1) Add it as the prerequisite of another file
+
+	Example::
+
+		#net/bpfilter/Makefile
+		userprogs := bpfilter_umh
+		$(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
+
+	$(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
+
+	(2) Use always-y
+
+	Example::
+
+		userprogs := binderfs_example
+		always-y := $(userprogs)
+
+	Kbuild provides the following shorthand for this:
+
+		userprogs-always-y := binderfs_example
+
+	This will tell Kbuild to build binderfs_example when it visits this
+	Makefile.
+
+6 Kbuild clean infrastructure
+=============================
+
+"make clean" deletes most generated files in the obj tree where the kernel
+is compiled. This includes generated files such as host programs.
+Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m),
+$(always-), $(extra-y), $(extra-) and $(targets). They are all deleted
+during "make clean". Files matching the patterns "*.[oas]", "*.ko", plus
+some additional files generated by kbuild are deleted all over the kernel
+source tree when "make clean" is executed.
+
+Additional files or directories can be specified in kbuild makefiles by use of
+$(clean-files).
+
+	Example::
+
+		#lib/Makefile
+		clean-files := crc32table.h
+
+When executing "make clean", the file "crc32table.h" will be deleted.
+Kbuild will assume files to be in the same relative directory as the
+Makefile, except if prefixed with $(objtree).
+
+To exclude certain files or directories from make clean, use the
+$(no-clean-files) variable.
+
+Usually kbuild descends down in subdirectories due to "obj-* := dir/",
+but in the architecture makefiles where the kbuild infrastructure
+is not sufficient this sometimes needs to be explicit.
+
+	Example::
+
+		#arch/x86/boot/Makefile
+		subdir- := compressed
+
+The above assignment instructs kbuild to descend down in the
+directory compressed/ when "make clean" is executed.
+
+To support the clean infrastructure in the Makefiles that build the
+final bootimage there is an optional target named archclean:
+
+	Example::
+
+		#arch/x86/Makefile
+		archclean:
+			$(Q)$(MAKE) $(clean)=arch/x86/boot
+
+When "make clean" is executed, make will descend down in arch/x86/boot,
+and clean as usual. The Makefile located in arch/x86/boot/ may use
+the subdir- trick to descend further down.
+
+Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
+included in the top level makefile, and the kbuild infrastructure
+is not operational at that point.
+
+Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
+be visited during "make clean".
+
+7 Architecture Makefiles
+========================
+
+The top level Makefile sets up the environment and does the preparation,
+before starting to descend down in the individual directories.
+The top level makefile contains the generic part, whereas
+arch/$(ARCH)/Makefile contains what is required to set up kbuild
+for said architecture.
+To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
+a few targets.
+
+When kbuild executes, the following steps are followed (roughly):
+
+1) Configuration of the kernel => produce .config
+2) Store kernel version in include/linux/version.h
+3) Updating all other prerequisites to the target prepare:
+   - Additional prerequisites are specified in arch/$(ARCH)/Makefile
+4) Recursively descend down in all directories listed in
+   init-* core* drivers-* net-* libs-* and build all targets.
+   - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
+5) All object files are then linked and the resulting file vmlinux is
+   located at the root of the obj tree.
+   The very first objects linked are listed in head-y, assigned by
+   arch/$(ARCH)/Makefile.
+6) Finally, the architecture-specific part does any required post processing
+   and builds the final bootimage.
+   - This includes building boot records
+   - Preparing initrd images and the like
+
+
+7.1 Set variables to tweak the build to the architecture
+--------------------------------------------------------
+
+    KBUILD_LDFLAGS
+	Generic $(LD) options
+
+	Flags used for all invocations of the linker.
+	Often specifying the emulation is sufficient.
+
+	Example::
+
+		#arch/s390/Makefile
+		KBUILD_LDFLAGS         := -m elf_s390
+
+	Note: ldflags-y can be used to further customise
+	the flags used. See section 3.7.
+
+    LDFLAGS_vmlinux
+	Options for $(LD) when linking vmlinux
+
+	LDFLAGS_vmlinux is used to specify additional flags to pass to
+	the linker when linking the final vmlinux image.
+	LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
+
+	Example::
+
+		#arch/x86/Makefile
+		LDFLAGS_vmlinux := -e stext
+
+    OBJCOPYFLAGS
+	objcopy flags
+
+	When $(call if_changed,objcopy) is used to translate a .o file,
+	the flags specified in OBJCOPYFLAGS will be used.
+	$(call if_changed,objcopy) is often used to generate raw binaries on
+	vmlinux.
+
+	Example::
+
+		#arch/s390/Makefile
+		OBJCOPYFLAGS := -O binary
+
+		#arch/s390/boot/Makefile
+		$(obj)/image: vmlinux FORCE
+			$(call if_changed,objcopy)
+
+	In this example, the binary $(obj)/image is a binary version of
+	vmlinux. The usage of $(call if_changed,xxx) will be described later.
+
+    KBUILD_AFLAGS
+	Assembler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Example::
+
+		#arch/sparc64/Makefile
+		KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
+
+    KBUILD_CFLAGS
+	$(CC) compiler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Often, the KBUILD_CFLAGS variable depends on the configuration.
+
+	Example::
+
+		#arch/x86/boot/compressed/Makefile
+		cflags-$(CONFIG_X86_32) := -march=i386
+		cflags-$(CONFIG_X86_64) := -mcmodel=small
+		KBUILD_CFLAGS += $(cflags-y)
+
+	Many arch Makefiles dynamically run the target C compiler to
+	probe supported options::
+
+		#arch/x86/Makefile
+
+		...
+		cflags-$(CONFIG_MPENTIUMII)     += $(call cc-option,\
+						-march=pentium2,-march=i686)
+		...
+		# Disable unit-at-a-time mode ...
+		KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
+		...
+
+
+	The first example utilises the trick that a config option expands
+	to 'y' when selected.
+
+    KBUILD_AFLAGS_KERNEL
+	Assembler options specific for built-in
+
+	$(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
+	resident kernel code.
+
+    KBUILD_AFLAGS_MODULE
+	Assembler options specific for modules
+
+	$(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
+	are used for assembler.
+
+	From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
+
+    KBUILD_CFLAGS_KERNEL
+	$(CC) options specific for built-in
+
+	$(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
+	resident kernel code.
+
+    KBUILD_CFLAGS_MODULE
+	Options for $(CC) when building modules
+
+	$(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
+	are used for $(CC).
+	From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
+
+    KBUILD_LDFLAGS_MODULE
+	Options for $(LD) when linking modules
+
+	$(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
+	used when linking modules. This is often a linker script.
+
+	From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).
+
+    KBUILD_LDS
+
+	The linker script with full path. Assigned by the top-level Makefile.
+
+    KBUILD_LDS_MODULE
+
+	The module linker script with full path. Assigned by the top-level
+	Makefile and additionally by the arch Makefile.
+
+    KBUILD_VMLINUX_OBJS
+
+	All object files for vmlinux. They are linked to vmlinux in the same
+	order as listed in KBUILD_VMLINUX_OBJS.
+
+    KBUILD_VMLINUX_LIBS
+
+	All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and
+	KBUILD_VMLINUX_LIBS together specify all the object files used to
+	link vmlinux.
+
+7.2 Add prerequisites to archheaders
+------------------------------------
+
+	The archheaders: rule is used to generate header files that
+	may be installed into user space by "make header_install".
+
+	It is run before "make archprepare" when run on the
+	architecture itself.
+
+
+7.3 Add prerequisites to archprepare
+------------------------------------
+
+	The archprepare: rule is used to list prerequisites that need to be
+	built before starting to descend down in the subdirectories.
+	This is usually used for header files containing assembler constants.
+
+	Example::
+
+		#arch/arm/Makefile
+		archprepare: maketools
+
+	In this example, the file target maketools will be processed
+	before descending down in the subdirectories.
+	See also chapter XXX-TODO that describes how kbuild supports
+	generating offset header files.
+
+
+7.4 List directories to visit when descending
+---------------------------------------------
+
+	An arch Makefile cooperates with the top Makefile to define variables
+	which specify how to build the vmlinux file.  Note that there is no
+	corresponding arch-specific section for modules; the module-building
+	machinery is all architecture-independent.
+
+
+	head-y, init-y, core-y, libs-y, drivers-y, net-y
+	    $(head-y) lists objects to be linked first in vmlinux.
+
+	    $(libs-y) lists directories where a lib.a archive can be located.
+
+	    The rest list directories where a built-in.a object file can be
+	    located.
+
+	    $(init-y) objects will be located after $(head-y).
+
+	    Then the rest follows in this order:
+
+		$(core-y), $(libs-y), $(drivers-y) and $(net-y).
+
+	    The top level Makefile defines values for all generic directories,
+	    and arch/$(ARCH)/Makefile only adds architecture-specific
+	    directories.
+
+	    Example::
+
+		#arch/sparc64/Makefile
+		core-y += arch/sparc64/kernel/
+		libs-y += arch/sparc64/prom/ arch/sparc64/lib/
+		drivers-$(CONFIG_OPROFILE)  += arch/sparc64/oprofile/
+
+
+7.5 Architecture-specific boot images
+-------------------------------------
+
+	An arch Makefile specifies goals that take the vmlinux file, compress
+	it, wrap it in bootstrapping code, and copy the resulting files
+	somewhere. This includes various kinds of installation commands.
+	The actual goals are not standardized across architectures.
+
+	It is common to locate any additional processing in a boot/
+	directory below arch/$(ARCH)/.
+
+	Kbuild does not provide any smart way to support building a
+	target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
+	call make manually to build a target in boot/.
+
+	The recommended approach is to include shortcuts in
+	arch/$(ARCH)/Makefile, and use the full path when calling down
+	into the arch/$(ARCH)/boot/Makefile.
+
+	Example::
+
+		#arch/x86/Makefile
+		boot := arch/x86/boot
+		bzImage: vmlinux
+			$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+
+	"$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
+	make in a subdirectory.
+
+	There are no rules for naming architecture-specific targets,
+	but executing "make help" will list all relevant targets.
+	To support this, $(archhelp) must be defined.
+
+	Example::
+
+		#arch/x86/Makefile
+		define archhelp
+		  echo  '* bzImage      - Image (arch/$(ARCH)/boot/bzImage)'
+		endif
+
+	When make is executed without arguments, the first goal encountered
+	will be built. In the top level Makefile the first goal present
+	is all:.
+	An architecture shall always, per default, build a bootable image.
+	In "make help", the default goal is highlighted with a '*'.
+	Add a new prerequisite to all: to select a default goal different
+	from vmlinux.
+
+	Example::
+
+		#arch/x86/Makefile
+		all: bzImage
+
+	When "make" is executed without arguments, bzImage will be built.
+
+7.6 Building non-kbuild targets
+-------------------------------
+
+    extra-y
+	extra-y specifies additional targets created in the current
+	directory, in addition to any targets specified by `obj-*`.
+
+	Listing all targets in extra-y is required for two purposes:
+
+	1) Enable kbuild to check changes in command lines
+
+	   - When $(call if_changed,xxx) is used
+
+	2) kbuild knows what files to delete during "make clean"
+
+	Example::
+
+		#arch/x86/kernel/Makefile
+		extra-y := head.o init_task.o
+
+	In this example, extra-y is used to list object files that
+	shall be built, but shall not be linked as part of built-in.a.
+
+7.7 Commands useful for building a boot image
+---------------------------------------------
+
+    Kbuild provides a few macros that are useful when building a
+    boot image.
+
+    if_changed
+	if_changed is the infrastructure used for the following commands.
+
+	Usage::
+
+		target: source(s) FORCE
+			$(call if_changed,ld/objcopy/gzip/...)
+
+	When the rule is evaluated, it is checked to see if any files
+	need an update, or the command line has changed since the last
+	invocation. The latter will force a rebuild if any options
+	to the executable have changed.
+	Any target that utilises if_changed must be listed in $(targets),
+	otherwise the command line check will fail, and the target will
+	always be built.
+	Assignments to $(targets) are without $(obj)/ prefix.
+	if_changed may be used in conjunction with custom commands as
+	defined in 7.8 "Custom kbuild commands".
+
+	Note: It is a typical mistake to forget the FORCE prerequisite.
+	Another common pitfall is that whitespace is sometimes
+	significant; for instance, the below will fail (note the extra space
+	after the comma)::
+
+		target: source(s) FORCE
+
+	**WRONG!**	$(call if_changed, ld/objcopy/gzip/...)
+
+        Note:
+	      if_changed should not be used more than once per target.
+              It stores the executed command in a corresponding .cmd
+
+        file and multiple calls would result in overwrites and
+        unwanted results when the target is up to date and only the
+        tests on changed commands trigger execution of commands.
+
+    ld
+	Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
+
+	Example::
+
+		#arch/x86/boot/Makefile
+		LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
+		LDFLAGS_setup    := -Ttext 0x0 -s --oformat binary -e begtext
+
+		targets += setup setup.o bootsect bootsect.o
+		$(obj)/setup $(obj)/bootsect: %: %.o FORCE
+			$(call if_changed,ld)
+
+	In this example, there are two possible targets, requiring different
+	options to the linker. The linker options are specified using the
+	LDFLAGS_$@ syntax - one for each potential target.
+	$(targets) are assigned all potential targets, by which kbuild knows
+	the targets and will:
+
+		1) check for commandline changes
+		2) delete target during make clean
+
+	The ": %: %.o" part of the prerequisite is a shorthand that
+	frees us from listing the setup.o and bootsect.o files.
+
+	Note:
+	      It is a common mistake to forget the "targets :=" assignment,
+	      resulting in the target file being recompiled for no
+	      obvious reason.
+
+    objcopy
+	Copy binary. Uses OBJCOPYFLAGS usually specified in
+	arch/$(ARCH)/Makefile.
+	OBJCOPYFLAGS_$@ may be used to set additional options.
+
+    gzip
+	Compress target. Use maximum compression to compress target.
+
+	Example::
+
+		#arch/x86/boot/compressed/Makefile
+		$(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
+			$(call if_changed,gzip)
+
+    dtc
+	Create flattened device tree blob object suitable for linking
+	into vmlinux. Device tree blobs linked into vmlinux are placed
+	in an init section in the image. Platform code *must* copy the
+	blob to non-init memory prior to calling unflatten_device_tree().
+
+	To use this command, simply add `*.dtb` into obj-y or targets, or make
+	some other target depend on `%.dtb`
+
+	A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
+	architecture Makefiles do no need to explicitly write out that rule.
+
+	Example::
+
+		targets += $(dtb-y)
+		DTC_FLAGS ?= -p 1024
+
+7.8 Custom kbuild commands
+--------------------------
+
+	When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
+	of a command is normally displayed.
+	To enable this behaviour for custom commands kbuild requires
+	two variables to be set::
+
+		quiet_cmd_<command>	- what shall be echoed
+		      cmd_<command>	- the command to execute
+
+	Example::
+
+		#
+		quiet_cmd_image = BUILD   $@
+		      cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
+		                                     $(obj)/vmlinux.bin > $@
+
+		targets += bzImage
+		$(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
+			$(call if_changed,image)
+			@echo 'Kernel: $@ is ready'
+
+	When updating the $(obj)/bzImage target, the line:
+
+		BUILD    arch/x86/boot/bzImage
+
+	will be displayed with "make KBUILD_VERBOSE=0".
+
+
+7.9 Preprocessing linker scripts
+--------------------------------
+
+	When the vmlinux image is built, the linker script
+	arch/$(ARCH)/kernel/vmlinux.lds is used.
+	The script is a preprocessed variant of the file vmlinux.lds.S
+	located in the same directory.
+	kbuild knows .lds files and includes a rule `*lds.S` -> `*lds`.
+
+	Example::
+
+		#arch/x86/kernel/Makefile
+		extra-y := vmlinux.lds
+
+		#Makefile
+		export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
+
+	The assignment to extra-y is used to tell kbuild to build the
+	target vmlinux.lds.
+	The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
+	specified options when building the target vmlinux.lds.
+
+	When building the `*.lds` target, kbuild uses the variables::
+
+		KBUILD_CPPFLAGS	: Set in top-level Makefile
+		cppflags-y	: May be set in the kbuild makefile
+		CPPFLAGS_$(@F)  : Target-specific flags.
+				Note that the full filename is used in this
+				assignment.
+
+	The kbuild infrastructure for `*lds` files is used in several
+	architecture-specific files.
+
+7.10 Generic header files
+-------------------------
+
+	The directory include/asm-generic contains the header files
+	that may be shared between individual architectures.
+	The recommended approach how to use a generic header file is
+	to list the file in the Kbuild file.
+	See "8.2 generic-y" for further info on syntax etc.
+
+7.11 Post-link pass
+-------------------
+
+	If the file arch/xxx/Makefile.postlink exists, this makefile
+	will be invoked for post-link objects (vmlinux and modules.ko)
+	for architectures to run post-link passes on. Must also handle
+	the clean target.
+
+	This pass runs after kallsyms generation. If the architecture
+	needs to modify symbol locations, rather than manipulate the
+	kallsyms, it may be easier to add another postlink target for
+	.tmp_vmlinux? targets to be called from link-vmlinux.sh.
+
+	For example, powerpc uses this to check relocation sanity of
+	the linked vmlinux file.
+
+8 Kbuild syntax for exported headers
+------------------------------------
+
+The kernel includes a set of headers that is exported to userspace.
+Many headers can be exported as-is but other headers require a
+minimal pre-processing before they are ready for user-space.
+The pre-processing does:
+
+- drop kernel-specific annotations
+- drop include of compiler.h
+- drop all sections that are kernel internal (guarded by `ifdef __KERNEL__`)
+
+All headers under include/uapi/, include/generated/uapi/,
+arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
+are exported.
+
+A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
+arch/<arch>/include/asm/ to list asm files coming from asm-generic.
+See subsequent chapter for the syntax of the Kbuild file.
+
+8.1 no-export-headers
+---------------------
+
+	no-export-headers is essentially used by include/uapi/linux/Kbuild to
+	avoid exporting specific headers (e.g. kvm.h) on architectures that do
+	not support it. It should be avoided as much as possible.
+
+8.2 generic-y
+-------------
+
+	If an architecture uses a verbatim copy of a header from
+	include/asm-generic then this is listed in the file
+	arch/$(ARCH)/include/asm/Kbuild like this:
+
+		Example::
+
+			#arch/x86/include/asm/Kbuild
+			generic-y += termios.h
+			generic-y += rtc.h
+
+	During the prepare phase of the build a wrapper include
+	file is generated in the directory::
+
+		arch/$(ARCH)/include/generated/asm
+
+	When a header is exported where the architecture uses
+	the generic header a similar wrapper is generated as part
+	of the set of exported headers in the directory::
+
+		usr/include/asm
+
+	The generated wrapper will in both cases look like the following:
+
+		Example: termios.h::
+
+			#include <asm-generic/termios.h>
+
+8.3 generated-y
+---------------
+
+	If an architecture generates other header files alongside generic-y
+	wrappers, generated-y specifies them.
+
+	This prevents them being treated as stale asm-generic wrappers and
+	removed.
+
+		Example::
+
+			#arch/x86/include/asm/Kbuild
+			generated-y += syscalls_32.h
+
+8.4 mandatory-y
+---------------
+
+	mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
+	to define the minimum set of ASM headers that all architectures must have.
+
+	This works like optional generic-y. If a mandatory header is missing
+	in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate
+	a wrapper of the asm-generic one.
+
+9 Kbuild Variables
+==================
+
+The top Makefile exports the following variables:
+
+    VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
+	These variables define the current kernel version.  A few arch
+	Makefiles actually use these values directly; they should use
+	$(KERNELRELEASE) instead.
+
+	$(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
+	three-part version number, such as "2", "4", and "0".  These three
+	values are always numeric.
+
+	$(EXTRAVERSION) defines an even tinier sublevel for pre-patches
+	or additional patches.	It is usually some non-numeric string
+	such as "-pre4", and is often blank.
+
+    KERNELRELEASE
+	$(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
+	for constructing installation directory names or showing in
+	version strings.  Some arch Makefiles use it for this purpose.
+
+    ARCH
+	This variable defines the target architecture, such as "i386",
+	"arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
+	determine which files to compile.
+
+	By default, the top Makefile sets $(ARCH) to be the same as the
+	host system architecture.  For a cross build, a user may
+	override the value of $(ARCH) on the command line::
+
+	    make ARCH=m68k ...
+
+
+    INSTALL_PATH
+	This variable defines a place for the arch Makefiles to install
+	the resident kernel image and System.map file.
+	Use this for architecture-specific install targets.
+
+    INSTALL_MOD_PATH, MODLIB
+	$(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
+	installation.  This variable is not defined in the Makefile but
+	may be passed in by the user if desired.
+
+	$(MODLIB) specifies the directory for module installation.
+	The top Makefile defines $(MODLIB) to
+	$(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE).  The user may
+	override this value on the command line if desired.
+
+    INSTALL_MOD_STRIP
+	If this variable is specified, it will cause modules to be stripped
+	after they are installed.  If INSTALL_MOD_STRIP is '1', then the
+	default option --strip-debug will be used.  Otherwise, the
+	INSTALL_MOD_STRIP value will be used as the option(s) to the strip
+	command.
+
+
+10 Makefile language
+====================
+
+The kernel Makefiles are designed to be run with GNU Make.  The Makefiles
+use only the documented features of GNU Make, but they do use many
+GNU extensions.
+
+GNU Make supports elementary list-processing functions.  The kernel
+Makefiles use a novel style of list building and manipulation with few
+"if" statements.
+
+GNU Make has two assignment operators, ":=" and "=".  ":=" performs
+immediate evaluation of the right-hand side and stores an actual string
+into the left-hand side.  "=" is like a formula definition; it stores the
+right-hand side in an unevaluated form and then evaluates this form each
+time the left-hand side is used.
+
+There are some cases where "=" is appropriate.  Usually, though, ":="
+is the right choice.
+
+11 Credits
+==========
+
+- Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
+- Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
+- Updates by Sam Ravnborg <sam@ravnborg.org>
+- Language QA by Jan Engelhardt <jengelh@gmx.de>
+
+12 TODO
+=======
+
+- Describe how kbuild supports shipped files with _shipped.
+- Generating offset header files.
+- Add more variables to chapters 7 or 9?
diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst
new file mode 100644
index 000000000..85ccc8788
--- /dev/null
+++ b/Documentation/kbuild/modules.rst
@@ -0,0 +1,561 @@
+=========================
+Building External Modules
+=========================
+
+This document describes how to build an out-of-tree kernel module.
+
+.. Table of Contents
+
+	=== 1 Introduction
+	=== 2 How to Build External Modules
+	   --- 2.1 Command Syntax
+	   --- 2.2 Options
+	   --- 2.3 Targets
+	   --- 2.4 Building Separate Files
+	=== 3. Creating a Kbuild File for an External Module
+	   --- 3.1 Shared Makefile
+	   --- 3.2 Separate Kbuild file and Makefile
+	   --- 3.3 Binary Blobs
+	   --- 3.4 Building Multiple Modules
+	=== 4. Include Files
+	   --- 4.1 Kernel Includes
+	   --- 4.2 Single Subdirectory
+	   --- 4.3 Several Subdirectories
+	=== 5. Module Installation
+	   --- 5.1 INSTALL_MOD_PATH
+	   --- 5.2 INSTALL_MOD_DIR
+	=== 6. Module Versioning
+	   --- 6.1 Symbols From the Kernel (vmlinux + modules)
+	   --- 6.2 Symbols and External Modules
+	   --- 6.3 Symbols From Another External Module
+	=== 7. Tips & Tricks
+	   --- 7.1 Testing for CONFIG_FOO_BAR
+
+
+
+1. Introduction
+===============
+
+"kbuild" is the build system used by the Linux kernel. Modules must use
+kbuild to stay compatible with changes in the build infrastructure and
+to pick up the right flags to "gcc." Functionality for building modules
+both in-tree and out-of-tree is provided. The method for building
+either is similar, and all modules are initially developed and built
+out-of-tree.
+
+Covered in this document is information aimed at developers interested
+in building out-of-tree (or "external") modules. The author of an
+external module should supply a makefile that hides most of the
+complexity, so one only has to type "make" to build the module. This is
+easily accomplished, and a complete example will be presented in
+section 3.
+
+
+2. How to Build External Modules
+================================
+
+To build external modules, you must have a prebuilt kernel available
+that contains the configuration and header files used in the build.
+Also, the kernel must have been built with modules enabled. If you are
+using a distribution kernel, there will be a package for the kernel you
+are running provided by your distribution.
+
+An alternative is to use the "make" target "modules_prepare." This will
+make sure the kernel contains the information required. The target
+exists solely as a simple way to prepare a kernel source tree for
+building external modules.
+
+NOTE: "modules_prepare" will not build Module.symvers even if
+CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
+executed to make module versioning work.
+
+2.1 Command Syntax
+==================
+
+	The command to build an external module is::
+
+		$ make -C <path_to_kernel_src> M=$PWD
+
+	The kbuild system knows that an external module is being built
+	due to the "M=<dir>" option given in the command.
+
+	To build against the running kernel use::
+
+		$ make -C /lib/modules/`uname -r`/build M=$PWD
+
+	Then to install the module(s) just built, add the target
+	"modules_install" to the command::
+
+		$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
+
+2.2 Options
+===========
+
+	($KDIR refers to the path of the kernel source directory.)
+
+	make -C $KDIR M=$PWD
+
+	-C $KDIR
+		The directory where the kernel source is located.
+		"make" will actually change to the specified directory
+		when executing and will change back when finished.
+
+	M=$PWD
+		Informs kbuild that an external module is being built.
+		The value given to "M" is the absolute path of the
+		directory where the external module (kbuild file) is
+		located.
+
+2.3 Targets
+===========
+
+	When building an external module, only a subset of the "make"
+	targets are available.
+
+	make -C $KDIR M=$PWD [target]
+
+	The default will build the module(s) located in the current
+	directory, so a target does not need to be specified. All
+	output files will also be generated in this directory. No
+	attempts are made to update the kernel source, and it is a
+	precondition that a successful "make" has been executed for the
+	kernel.
+
+	modules
+		The default target for external modules. It has the
+		same functionality as if no target was specified. See
+		description above.
+
+	modules_install
+		Install the external module(s). The default location is
+		/lib/modules/<kernel_release>/extra/, but a prefix may
+		be added with INSTALL_MOD_PATH (discussed in section 5).
+
+	clean
+		Remove all generated files in the module directory only.
+
+	help
+		List the available targets for external modules.
+
+2.4 Building Separate Files
+===========================
+
+	It is possible to build single files that are part of a module.
+	This works equally well for the kernel, a module, and even for
+	external modules.
+
+	Example (The module foo.ko, consist of bar.o and baz.o)::
+
+		make -C $KDIR M=$PWD bar.lst
+		make -C $KDIR M=$PWD baz.o
+		make -C $KDIR M=$PWD foo.ko
+		make -C $KDIR M=$PWD ./
+
+
+3. Creating a Kbuild File for an External Module
+================================================
+
+In the last section we saw the command to build a module for the
+running kernel. The module is not actually built, however, because a
+build file is required. Contained in this file will be the name of
+the module(s) being built, along with the list of requisite source
+files. The file may be as simple as a single line::
+
+	obj-m := <module_name>.o
+
+The kbuild system will build <module_name>.o from <module_name>.c,
+and, after linking, will result in the kernel module <module_name>.ko.
+The above line can be put in either a "Kbuild" file or a "Makefile."
+When the module is built from multiple sources, an additional line is
+needed listing the files::
+
+	<module_name>-y := <src1>.o <src2>.o ...
+
+NOTE: Further documentation describing the syntax used by kbuild is
+located in Documentation/kbuild/makefiles.rst.
+
+The examples below demonstrate how to create a build file for the
+module 8123.ko, which is built from the following files::
+
+	8123_if.c
+	8123_if.h
+	8123_pci.c
+	8123_bin.o_shipped	<= Binary blob
+
+3.1 Shared Makefile
+-------------------
+
+	An external module always includes a wrapper makefile that
+	supports building the module using "make" with no arguments.
+	This target is not used by kbuild; it is only for convenience.
+	Additional functionality, such as test targets, can be included
+	but should be filtered out from kbuild due to possible name
+	clashes.
+
+	Example 1::
+
+		--> filename: Makefile
+		ifneq ($(KERNELRELEASE),)
+		# kbuild part of makefile
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		else
+		# normal makefile
+		KDIR ?= /lib/modules/`uname -r`/build
+
+		default:
+			$(MAKE) -C $(KDIR) M=$$PWD
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin.o_shipped
+
+		endif
+
+	The check for KERNELRELEASE is used to separate the two parts
+	of the makefile. In the example, kbuild will only see the two
+	assignments, whereas "make" will see everything except these
+	two assignments. This is due to two passes made on the file:
+	the first pass is by the "make" instance run on the command
+	line; the second pass is by the kbuild system, which is
+	initiated by the parameterized "make" in the default target.
+
+3.2 Separate Kbuild File and Makefile
+-------------------------------------
+
+	In newer versions of the kernel, kbuild will first look for a
+	file named "Kbuild," and only if that is not found, will it
+	then look for a makefile. Utilizing a "Kbuild" file allows us
+	to split up the makefile from example 1 into two files:
+
+	Example 2::
+
+		--> filename: Kbuild
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		--> filename: Makefile
+		KDIR ?= /lib/modules/`uname -r`/build
+
+		default:
+			$(MAKE) -C $(KDIR) M=$$PWD
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin.o_shipped
+
+	The split in example 2 is questionable due to the simplicity of
+	each file; however, some external modules use makefiles
+	consisting of several hundred lines, and here it really pays
+	off to separate the kbuild part from the rest.
+
+	The next example shows a backward compatible version.
+
+	Example 3::
+
+		--> filename: Kbuild
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		--> filename: Makefile
+		ifneq ($(KERNELRELEASE),)
+		# kbuild part of makefile
+		include Kbuild
+
+		else
+		# normal makefile
+		KDIR ?= /lib/modules/`uname -r`/build
+
+		default:
+			$(MAKE) -C $(KDIR) M=$$PWD
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin.o_shipped
+
+		endif
+
+	Here the "Kbuild" file is included from the makefile. This
+	allows an older version of kbuild, which only knows of
+	makefiles, to be used when the "make" and kbuild parts are
+	split into separate files.
+
+3.3 Binary Blobs
+----------------
+
+	Some external modules need to include an object file as a blob.
+	kbuild has support for this, but requires the blob file to be
+	named <filename>_shipped. When the kbuild rules kick in, a copy
+	of <filename>_shipped is created with _shipped stripped off,
+	giving us <filename>. This shortened filename can be used in
+	the assignment to the module.
+
+	Throughout this section, 8123_bin.o_shipped has been used to
+	build the kernel module 8123.ko; it has been included as
+	8123_bin.o::
+
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+	Although there is no distinction between the ordinary source
+	files and the binary file, kbuild will pick up different rules
+	when creating the object file for the module.
+
+3.4 Building Multiple Modules
+=============================
+
+	kbuild supports building multiple modules with a single build
+	file. For example, if you wanted to build two modules, foo.ko
+	and bar.ko, the kbuild lines would be::
+
+		obj-m := foo.o bar.o
+		foo-y := <foo_srcs>
+		bar-y := <bar_srcs>
+
+	It is that simple!
+
+
+4. Include Files
+================
+
+Within the kernel, header files are kept in standard locations
+according to the following rule:
+
+	* If the header file only describes the internal interface of a
+	  module, then the file is placed in the same directory as the
+	  source files.
+	* If the header file describes an interface used by other parts
+	  of the kernel that are located in different directories, then
+	  the file is placed in include/linux/.
+
+	  NOTE:
+	      There are two notable exceptions to this rule: larger
+	      subsystems have their own directory under include/, such as
+	      include/scsi; and architecture specific headers are located
+	      under arch/$(ARCH)/include/.
+
+4.1 Kernel Includes
+-------------------
+
+	To include a header file located under include/linux/, simply
+	use::
+
+		#include <linux/module.h>
+
+	kbuild will add options to "gcc" so the relevant directories
+	are searched.
+
+4.2 Single Subdirectory
+-----------------------
+
+	External modules tend to place header files in a separate
+	include/ directory where their source is located, although this
+	is not the usual kernel style. To inform kbuild of the
+	directory, use either ccflags-y or CFLAGS_<filename>.o.
+
+	Using the example from section 3, if we moved 8123_if.h to a
+	subdirectory named include, the resulting kbuild file would
+	look like::
+
+		--> filename: Kbuild
+		obj-m := 8123.o
+
+		ccflags-y := -Iinclude
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+	Note that in the assignment there is no space between -I and
+	the path. This is a limitation of kbuild: there must be no
+	space present.
+
+4.3 Several Subdirectories
+--------------------------
+
+	kbuild can handle files that are spread over several directories.
+	Consider the following example::
+
+		.
+		|__ src
+		|   |__ complex_main.c
+		|   |__ hal
+		|	|__ hardwareif.c
+		|	|__ include
+		|	    |__ hardwareif.h
+		|__ include
+		|__ complex.h
+
+	To build the module complex.ko, we then need the following
+	kbuild file::
+
+		--> filename: Kbuild
+		obj-m := complex.o
+		complex-y := src/complex_main.o
+		complex-y += src/hal/hardwareif.o
+
+		ccflags-y := -I$(src)/include
+		ccflags-y += -I$(src)/src/hal/include
+
+	As you can see, kbuild knows how to handle object files located
+	in other directories. The trick is to specify the directory
+	relative to the kbuild file's location. That being said, this
+	is NOT recommended practice.
+
+	For the header files, kbuild must be explicitly told where to
+	look. When kbuild executes, the current directory is always the
+	root of the kernel tree (the argument to "-C") and therefore an
+	absolute path is needed. $(src) provides the absolute path by
+	pointing to the directory where the currently executing kbuild
+	file is located.
+
+
+5. Module Installation
+======================
+
+Modules which are included in the kernel are installed in the
+directory:
+
+	/lib/modules/$(KERNELRELEASE)/kernel/
+
+And external modules are installed in:
+
+	/lib/modules/$(KERNELRELEASE)/extra/
+
+5.1 INSTALL_MOD_PATH
+--------------------
+
+	Above are the default directories but as always some level of
+	customization is possible. A prefix can be added to the
+	installation path using the variable INSTALL_MOD_PATH::
+
+		$ make INSTALL_MOD_PATH=/frodo modules_install
+		=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
+
+	INSTALL_MOD_PATH may be set as an ordinary shell variable or,
+	as shown above, can be specified on the command line when
+	calling "make." This has effect when installing both in-tree
+	and out-of-tree modules.
+
+5.2 INSTALL_MOD_DIR
+-------------------
+
+	External modules are by default installed to a directory under
+	/lib/modules/$(KERNELRELEASE)/extra/, but you may wish to
+	locate modules for a specific functionality in a separate
+	directory. For this purpose, use INSTALL_MOD_DIR to specify an
+	alternative name to "extra."::
+
+		$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
+		       M=$PWD modules_install
+		=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/
+
+
+6. Module Versioning
+====================
+
+Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
+as a simple ABI consistency check. A CRC value of the full prototype
+for an exported symbol is created. When a module is loaded/used, the
+CRC values contained in the kernel are compared with similar values in
+the module; if they are not equal, the kernel refuses to load the
+module.
+
+Module.symvers contains a list of all exported symbols from a kernel
+build.
+
+6.1 Symbols From the Kernel (vmlinux + modules)
+-----------------------------------------------
+
+	During a kernel build, a file named Module.symvers will be
+	generated. Module.symvers contains all exported symbols from
+	the kernel and compiled modules. For each symbol, the
+	corresponding CRC value is also stored.
+
+	The syntax of the Module.symvers file is::
+
+		<CRC>       <Symbol>         <Module>                         <Export Type>     <Namespace>
+
+		0xe1cc2a05  usb_stor_suspend drivers/usb/storage/usb-storage  EXPORT_SYMBOL_GPL USB_STORAGE
+
+	The fields are separated by tabs and values may be empty (e.g.
+	if no namespace is defined for an exported symbol).
+
+	For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
+	would read 0x00000000.
+
+	Module.symvers serves two purposes:
+
+	1) It lists all exported symbols from vmlinux and all modules.
+	2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
+
+6.2 Symbols and External Modules
+--------------------------------
+
+	When building an external module, the build system needs access
+	to the symbols from the kernel to check if all external symbols
+	are defined. This is done in the MODPOST step. modpost obtains
+	the symbols by reading Module.symvers from the kernel source
+	tree. During the MODPOST step, a new Module.symvers file will be
+	written containing all exported symbols from that external module.
+
+6.3 Symbols From Another External Module
+----------------------------------------
+
+	Sometimes, an external module uses exported symbols from
+	another external module. Kbuild needs to have full knowledge of
+	all symbols to avoid spitting out warnings about undefined
+	symbols. Two solutions exist for this situation.
+
+	NOTE: The method with a top-level kbuild file is recommended
+	but may be impractical in certain situations.
+
+	Use a top-level kbuild file
+		If you have two modules, foo.ko and bar.ko, where
+		foo.ko needs symbols from bar.ko, you can use a
+		common top-level kbuild file so both modules are
+		compiled in the same build. Consider the following
+		directory layout::
+
+			./foo/ <= contains foo.ko
+			./bar/ <= contains bar.ko
+
+		The top-level kbuild file would then look like::
+
+			#./Kbuild (or ./Makefile):
+				obj-m := foo/ bar/
+
+		And executing::
+
+			$ make -C $KDIR M=$PWD
+
+		will then do the expected and compile both modules with
+		full knowledge of symbols from either module.
+
+	Use "make" variable KBUILD_EXTRA_SYMBOLS
+		If it is impractical to add a top-level kbuild file,
+		you can assign a space separated list
+		of files to KBUILD_EXTRA_SYMBOLS in your build file.
+		These files will be loaded by modpost during the
+		initialization of its symbol tables.
+
+
+7. Tips & Tricks
+================
+
+7.1 Testing for CONFIG_FOO_BAR
+------------------------------
+
+	Modules often need to check for certain `CONFIG_` options to
+	decide if a specific feature is included in the module. In
+	kbuild this is done by referencing the `CONFIG_` variable
+	directly::
+
+		#fs/ext2/Makefile
+		obj-$(CONFIG_EXT2_FS) += ext2.o
+
+		ext2-y := balloc.o bitmap.o dir.o
+		ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
+
+	External modules have traditionally used "grep" to check for
+	specific `CONFIG_` settings directly in .config. This usage is
+	broken. As introduced before, external modules should use
+	kbuild for building and can therefore use the same methods as
+	in-tree modules when testing for `CONFIG_` definitions.
diff --git a/Documentation/kbuild/reproducible-builds.rst b/Documentation/kbuild/reproducible-builds.rst
new file mode 100644
index 000000000..3b25655e4
--- /dev/null
+++ b/Documentation/kbuild/reproducible-builds.rst
@@ -0,0 +1,127 @@
+===================
+Reproducible builds
+===================
+
+It is generally desirable that building the same source code with
+the same set of tools is reproducible, i.e. the output is always
+exactly the same.  This makes it possible to verify that the build
+infrastructure for a binary distribution or embedded system has not
+been subverted.  This can also make it easier to verify that a source
+or tool change does not make any difference to the resulting binaries.
+
+The `Reproducible Builds project`_ has more information about this
+general topic.  This document covers the various reasons why building
+the kernel may be unreproducible, and how to avoid them.
+
+Timestamps
+----------
+
+The kernel embeds timestamps in three places:
+
+* The version string exposed by ``uname()`` and included in
+  ``/proc/version``
+
+* File timestamps in the embedded initramfs
+
+* If enabled via ``CONFIG_IKHEADERS``, file timestamps of kernel
+  headers embedded in the kernel or respective module,
+  exposed via ``/sys/kernel/kheaders.tar.xz``
+
+By default the timestamp is the current time and in the case of
+``kheaders`` the various files' modification times. This must
+be overridden using the `KBUILD_BUILD_TIMESTAMP`_ variable.
+If you are building from a git commit, you could use its commit date.
+
+The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
+and enables warnings if they are used.  If you incorporate external
+code that does use these, you must override the timestamp they
+correspond to by setting the `SOURCE_DATE_EPOCH`_ environment
+variable.
+
+User, host
+----------
+
+The kernel embeds the building user and host names in
+``/proc/version``.  These must be overridden using the
+`KBUILD_BUILD_USER and KBUILD_BUILD_HOST`_ variables.  If you are
+building from a git commit, you could use its committer address.
+
+Absolute filenames
+------------------
+
+When the kernel is built out-of-tree, debug information may include
+absolute filenames for the source files.  This must be overridden by
+including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable.
+
+Depending on the compiler used, the ``__FILE__`` macro may also expand
+to an absolute filename in an out-of-tree build.  Kbuild automatically
+uses the ``-fmacro-prefix-map`` option to prevent this, if it is
+supported.
+
+The Reproducible Builds web site has more information about these
+`prefix-map options`_.
+
+Generated files in source packages
+----------------------------------
+
+The build processes for some programs under the ``tools/``
+subdirectory do not completely support out-of-tree builds.  This may
+cause a later source package build using e.g. ``make rpm-pkg`` to
+include generated files.  You should ensure the source tree is
+pristine by running ``make mrproper`` or ``git clean -d -f -x`` before
+building a source package.
+
+Module signing
+--------------
+
+If you enable ``CONFIG_MODULE_SIG_ALL``, the default behaviour is to
+generate a different temporary key for each build, resulting in the
+modules being unreproducible.  However, including a signing key with
+your source would presumably defeat the purpose of signing modules.
+
+One approach to this is to divide up the build process so that the
+unreproducible parts can be treated as sources:
+
+1. Generate a persistent signing key.  Add the certificate for the key
+   to the kernel source.
+
+2. Set the ``CONFIG_SYSTEM_TRUSTED_KEYS`` symbol to include the
+   signing key's certificate, set ``CONFIG_MODULE_SIG_KEY`` to an
+   empty string, and disable ``CONFIG_MODULE_SIG_ALL``.
+   Build the kernel and modules.
+
+3. Create detached signatures for the modules, and publish them as
+   sources.
+
+4. Perform a second build that attaches the module signatures.  It
+   can either rebuild the modules or use the output of step 2.
+
+Structure randomisation
+-----------------------
+
+If you enable ``CONFIG_GCC_PLUGIN_RANDSTRUCT``, you will need to
+pre-generate the random seed in
+``scripts/gcc-plugins/randomize_layout_seed.h`` so the same value
+is used in rebuilds.
+
+Debug info conflicts
+--------------------
+
+This is not a problem of unreproducibility, but of generated files
+being *too* reproducible.
+
+Once you set all the necessary variables for a reproducible build, a
+vDSO's debug information may be identical even for different kernel
+versions.  This can result in file conflicts between debug information
+packages for the different kernel versions.
+
+To avoid this, you can make the vDSO different for different
+kernel versions by including an arbitrary string of "salt" in it.
+This is specified by the Kconfig symbol ``CONFIG_BUILD_SALT``.
+
+.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
+.. _KBUILD_BUILD_USER and KBUILD_BUILD_HOST: kbuild.html#kbuild-build-user-kbuild-build-host
+.. _KCFLAGS: kbuild.html#kcflags
+.. _prefix-map options: https://reproducible-builds.org/docs/build-path/
+.. _Reproducible Builds project: https://reproducible-builds.org/
+.. _SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/