Adding upstream version 4.19.249.upstream/4.19.249upstream

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

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+===================
+Userland interfaces
+===================
+
+The DRM core exports several interfaces to applications, generally
+intended to be used through corresponding libdrm wrapper functions. In
+addition, drivers export device-specific interfaces for use by userspace
+drivers & device-aware applications through ioctls and sysfs files.
+
+External interfaces include: memory mapping, context management, DMA
+operations, AGP management, vblank control, fence management, memory
+management, and output management.
+
+Cover generic ioctls and sysfs layout here. We only need high-level
+info, since man pages should cover the rest.
+
+libdrm Device Lookup
+====================
+
+.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
+   :doc: getunique and setversion story
+
+
+.. _drm_primary_node:
+
+Primary Nodes, DRM Master and Authentication
+============================================
+
+.. kernel-doc:: drivers/gpu/drm/drm_auth.c
+   :doc: master and authentication
+
+.. kernel-doc:: drivers/gpu/drm/drm_auth.c
+   :export:
+
+.. kernel-doc:: include/drm/drm_auth.h
+   :internal:
+
+Open-Source Userspace Requirements
+==================================
+
+The DRM subsystem has stricter requirements than most other kernel subsystems on
+what the userspace side for new uAPI needs to look like. This section here
+explains what exactly those requirements are, and why they exist.
+
+The short summary is that any addition of DRM uAPI requires corresponding
+open-sourced userspace patches, and those patches must be reviewed and ready for
+merging into a suitable and canonical upstream project.
+
+GFX devices (both display and render/GPU side) are really complex bits of
+hardware, with userspace and kernel by necessity having to work together really
+closely.  The interfaces, for rendering and modesetting, must be extremely wide
+and flexible, and therefore it is almost always impossible to precisely define
+them for every possible corner case. This in turn makes it really practically
+infeasible to differentiate between behaviour that's required by userspace, and
+which must not be changed to avoid regressions, and behaviour which is only an
+accidental artifact of the current implementation.
+
+Without access to the full source code of all userspace users that means it
+becomes impossible to change the implementation details, since userspace could
+depend upon the accidental behaviour of the current implementation in minute
+details. And debugging such regressions without access to source code is pretty
+much impossible. As a consequence this means:
+
+- The Linux kernel's "no regression" policy holds in practice only for
+  open-source userspace of the DRM subsystem. DRM developers are perfectly fine
+  if closed-source blob drivers in userspace use the same uAPI as the open
+  drivers, but they must do so in the exact same way as the open drivers.
+  Creative (ab)use of the interfaces will, and in the past routinely has, lead
+  to breakage.
+
+- Any new userspace interface must have an open-source implementation as
+  demonstration vehicle.
+
+The other reason for requiring open-source userspace is uAPI review. Since the
+kernel and userspace parts of a GFX stack must work together so closely, code
+review can only assess whether a new interface achieves its goals by looking at
+both sides. Making sure that the interface indeed covers the use-case fully
+leads to a few additional requirements:
+
+- The open-source userspace must not be a toy/test application, but the real
+  thing. Specifically it needs to handle all the usual error and corner cases.
+  These are often the places where new uAPI falls apart and hence essential to
+  assess the fitness of a proposed interface.
+
+- The userspace side must be fully reviewed and tested to the standards of that
+  userspace project. For e.g. mesa this means piglit testcases and review on the
+  mailing list. This is again to ensure that the new interface actually gets the
+  job done.
+
+- The userspace patches must be against the canonical upstream, not some vendor
+  fork. This is to make sure that no one cheats on the review and testing
+  requirements by doing a quick fork.
+
+- The kernel patch can only be merged after all the above requirements are met,
+  but it **must** be merged **before** the userspace patches land. uAPI always flows
+  from the kernel, doing things the other way round risks divergence of the uAPI
+  definitions and header files.
+
+These are fairly steep requirements, but have grown out from years of shared
+pain and experience with uAPI added hastily, and almost always regretted about
+just as fast. GFX devices change really fast, requiring a paradigm shift and
+entire new set of uAPI interfaces every few years at least. Together with the
+Linux kernel's guarantee to keep existing userspace running for 10+ years this
+is already rather painful for the DRM subsystem, with multiple different uAPIs
+for the same thing co-existing. If we add a few more complete mistakes into the
+mix every year it would be entirely unmanageable.
+
+.. _drm_render_node:
+
+Render nodes
+============
+
+DRM core provides multiple character-devices for user-space to use.
+Depending on which device is opened, user-space can perform a different
+set of operations (mainly ioctls). The primary node is always created
+and called card<num>. Additionally, a currently unused control node,
+called controlD<num> is also created. The primary node provides all
+legacy operations and historically was the only interface used by
+userspace. With KMS, the control node was introduced. However, the
+planned KMS control interface has never been written and so the control
+node stays unused to date.
+
+With the increased use of offscreen renderers and GPGPU applications,
+clients no longer require running compositors or graphics servers to
+make use of a GPU. But the DRM API required unprivileged clients to
+authenticate to a DRM-Master prior to getting GPU access. To avoid this
+step and to grant clients GPU access without authenticating, render
+nodes were introduced. Render nodes solely serve render clients, that
+is, no modesetting or privileged ioctls can be issued on render nodes.
+Only non-global rendering commands are allowed. If a driver supports
+render nodes, it must advertise it via the DRIVER_RENDER DRM driver
+capability. If not supported, the primary node must be used for render
+clients together with the legacy drmAuth authentication procedure.
+
+If a driver advertises render node support, DRM core will create a
+separate render node called renderD<num>. There will be one render node
+per device. No ioctls except PRIME-related ioctls will be allowed on
+this node. Especially GEM_OPEN will be explicitly prohibited. Render
+nodes are designed to avoid the buffer-leaks, which occur if clients
+guess the flink names or mmap offsets on the legacy interface.
+Additionally to this basic interface, drivers must mark their
+driver-dependent render-only ioctls as DRM_RENDER_ALLOW so render
+clients can use them. Driver authors must be careful not to allow any
+privileged ioctls on render nodes.
+
+With render nodes, user-space can now control access to the render node
+via basic file-system access-modes. A running graphics server which
+authenticates clients on the privileged primary/legacy node is no longer
+required. Instead, a client can open the render node and is immediately
+granted GPU access. Communication between clients (or servers) is done
+via PRIME. FLINK from render node to legacy node is not supported. New
+clients must not use the insecure FLINK interface.
+
+Besides dropping all modeset/global ioctls, render nodes also drop the
+DRM-Master concept. There is no reason to associate render clients with
+a DRM-Master as they are independent of any graphics server. Besides,
+they must work without any running master, anyway. Drivers must be able
+to run without a master object if they support render nodes. If, on the
+other hand, a driver requires shared state between clients which is
+visible to user-space and accessible beyond open-file boundaries, they
+cannot support render nodes.
+
+.. _drm_driver_ioctl:
+
+IOCTL Support on Device Nodes
+=============================
+
+.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
+   :doc: driver specific ioctls
+
+Recommended IOCTL Return Values
+-------------------------------
+
+In theory a driver's IOCTL callback is only allowed to return very few error
+codes. In practice it's good to abuse a few more. This section documents common
+practice within the DRM subsystem:
+
+ENOENT:
+        Strictly this should only be used when a file doesn't exist e.g. when
+        calling the open() syscall. We reuse that to signal any kind of object
+        lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS
+        object handles and similar cases.
+
+ENOSPC:
+        Some drivers use this to differentiate "out of kernel memory" from "out
+        of VRAM". Sometimes also applies to other limited gpu resources used for
+        rendering (e.g. when you have a special limited compression buffer).
+        Sometimes resource allocation/reservation issues in command submission
+        IOCTLs are also signalled through EDEADLK.
+
+        Simply running out of kernel/system memory is signalled through ENOMEM.
+
+EPERM/EACCESS:
+        Returned for an operation that is valid, but needs more privileges.
+        E.g. root-only or much more common, DRM master-only operations return
+        this when when called by unpriviledged clients. There's no clear
+        difference between EACCESS and EPERM.
+
+ENODEV:
+        Feature (like PRIME, modesetting, GEM) is not supported by the driver.
+
+ENXIO:
+        Remote failure, either a hardware transaction (like i2c), but also used
+        when the exporting driver of a shared dma-buf or fence doesn't support a
+        feature needed.
+
+EINTR:
+        DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can
+        return EINTR and in such a case should be restarted with the IOCTL
+        parameters left unchanged.
+
+EIO:
+        The GPU died and couldn't be resurrected through a reset. Modesetting
+        hardware failures are signalled through the "link status" connector
+        property.
+
+EINVAL:
+        Catch-all for anything that is an invalid argument combination which
+        cannot work.
+
+IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their
+usage is in line with the common meanings. The above list tries to just document
+DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of
+"this IOCTL does not exist", and is used exactly as such in DRM.
+
+.. kernel-doc:: include/drm/drm_ioctl.h
+   :internal:
+
+.. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
+   :export:
+
+.. kernel-doc:: drivers/gpu/drm/drm_ioc32.c
+   :export:
+
+Testing and validation
+======================
+
+Validating changes with IGT
+---------------------------
+
+There's a collection of tests that aims to cover the whole functionality of
+DRM drivers and that can be used to check that changes to DRM drivers or the
+core don't regress existing functionality. This test suite is called IGT and
+its code can be found in https://cgit.freedesktop.org/drm/igt-gpu-tools/.
+
+To build IGT, start by installing its build dependencies. In Debian-based
+systems::
+
+	# apt-get build-dep intel-gpu-tools
+
+And in Fedora-based systems::
+
+	# dnf builddep intel-gpu-tools
+
+Then clone the repository::
+
+	$ git clone git://anongit.freedesktop.org/drm/igt-gpu-tools
+
+Configure the build system and start the build::
+
+	$ cd igt-gpu-tools && ./autogen.sh && make -j6
+
+Download the piglit dependency::
+
+	$ ./scripts/run-tests.sh -d
+
+And run the tests::
+
+	$ ./scripts/run-tests.sh -t kms -t core -s
+
+run-tests.sh is a wrapper around piglit that will execute the tests matching
+the -t options. A report in HTML format will be available in
+./results/html/index.html. Results can be compared with piglit.
+
+Display CRC Support
+-------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
+   :doc: CRC ABI
+
+.. kernel-doc:: drivers/gpu/drm/drm_debugfs_crc.c
+   :export:
+
+Debugfs Support
+---------------
+
+.. kernel-doc:: include/drm/drm_debugfs.h
+   :internal:
+
+.. kernel-doc:: drivers/gpu/drm/drm_debugfs.c
+   :export:
+
+Sysfs Support
+=============
+
+.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
+   :doc: overview
+
+.. kernel-doc:: drivers/gpu/drm/drm_sysfs.c
+   :export:
+
+
+VBlank event handling
+=====================
+
+The DRM core exposes two vertical blank related ioctls:
+
+DRM_IOCTL_WAIT_VBLANK
+    This takes a struct drm_wait_vblank structure as its argument, and
+    it is used to block or request a signal when a specified vblank
+    event occurs.
+
+DRM_IOCTL_MODESET_CTL
+    This was only used for user-mode-settind drivers around modesetting
+    changes to allow the kernel to update the vblank interrupt after
+    mode setting, since on many devices the vertical blank counter is
+    reset to 0 at some point during modeset. Modern drivers should not
+    call this any more since with kernel mode setting it is a no-op.