Adding upstream version 4.19.249.upstream/4.19.249

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

1 files changed, 100 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg b/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
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+What:		/sys/firmware/qemu_fw_cfg/
+Date:		August 2015
+Contact:	Gabriel Somlo <somlo@cmu.edu>
+Description:
+		Several different architectures supported by QEMU (x86, arm,
+		sun4*, ppc/mac) are provisioned with a firmware configuration
+		(fw_cfg) device, originally intended as a way for the host to
+		provide configuration data to the guest firmware. Starting
+		with QEMU v2.4, arbitrary fw_cfg file entries may be specified
+		by the user on the command line, which makes fw_cfg additionally
+		useful as an out-of-band, asynchronous mechanism for providing
+		configuration data to the guest userspace.
+
+		The authoritative guest-side hardware interface documentation
+		to the fw_cfg device can be found in "docs/specs/fw_cfg.txt"
+		in the QEMU source tree.
+
+		=== SysFS fw_cfg Interface ===
+
+		The fw_cfg sysfs interface described in this document is only
+		intended to display discoverable blobs (i.e., those registered
+		with the file directory), as there is no way to determine the
+		presence or size of "legacy" blobs (with selector keys between
+		0x0002 and 0x0018) programmatically.
+
+		All fw_cfg information is shown under:
+
+			/sys/firmware/qemu_fw_cfg/
+
+		The only legacy blob displayed is the fw_cfg device revision:
+
+			/sys/firmware/qemu_fw_cfg/rev
+
+		--- Discoverable fw_cfg blobs by selector key ---
+
+		All discoverable blobs listed in the fw_cfg file directory are
+		displayed as entries named after their unique selector key
+		value, e.g.:
+
+			/sys/firmware/qemu_fw_cfg/by_key/32
+			/sys/firmware/qemu_fw_cfg/by_key/33
+			/sys/firmware/qemu_fw_cfg/by_key/34
+			...
+
+		Each such fw_cfg sysfs entry has the following values exported
+		as attributes:
+
+		name  	: The 56-byte nul-terminated ASCII string used as the
+			  blob's 'file name' in the fw_cfg directory.
+		size  	: The length of the blob, as given in the fw_cfg
+			  directory.
+		key	: The value of the blob's selector key as given in the
+			  fw_cfg directory. This value is the same as used in
+			  the parent directory name.
+		raw	: The raw bytes of the blob, obtained by selecting the
+			  entry via the control register, and reading a number
+			  of bytes equal to the blob size from the data
+			  register.
+
+		--- Listing fw_cfg blobs by file name ---
+
+		While the fw_cfg device does not impose any specific naming
+		convention on the blobs registered in the file directory,
+		QEMU developers have traditionally used path name semantics
+		to give each blob a descriptive name. For example:
+
+			"bootorder"
+			"genroms/kvmvapic.bin"
+			"etc/e820"
+			"etc/boot-fail-wait"
+			"etc/system-states"
+			"etc/table-loader"
+			"etc/acpi/rsdp"
+			"etc/acpi/tables"
+			"etc/smbios/smbios-tables"
+			"etc/smbios/smbios-anchor"
+			...
+
+		In addition to the listing by unique selector key described
+		above, the fw_cfg sysfs driver also attempts to build a tree
+		of directories matching the path name components of fw_cfg
+		blob names, ending in symlinks to the by_key entry for each
+		"basename", as illustrated below (assume current directory is
+		/sys/firmware):
+
+		    qemu_fw_cfg/by_name/bootorder -> ../by_key/38
+		    qemu_fw_cfg/by_name/etc/e820 -> ../../by_key/35
+		    qemu_fw_cfg/by_name/etc/acpi/rsdp -> ../../../by_key/41
+		    ...
+
+		Construction of the directory tree and symlinks is done on a
+		"best-effort" basis, as there is no guarantee that components
+		of fw_cfg blob names are always "well behaved". I.e., there is
+		the possibility that a symlink (basename) will conflict with
+		a dirname component of another fw_cfg blob, in which case the
+		creation of the offending /sys/firmware/qemu_fw_cfg/by_name
+		entry will be skipped.
+
+		The authoritative list of entries will continue to be found
+		under the /sys/firmware/qemu_fw_cfg/by_key directory.