firmware: introduce sysfs driver for QEMU's fw_cfg device

Make fw_cfg entries of type "file" available via sysfs. Entries
are listed under /sys/firmware/qemu_fw_cfg/by_key, in folders
named after each entry's selector key. Filename, selector value,
and size read-only attributes are included for each entry. Also,
a "raw" attribute allows retrieval of the full binary content of
each entry.

The fw_cfg device can be instantiated automatically from ACPI or
the Device Tree, or manually by using a kernel module (or command
line) parameter, with a syntax outlined in the documentation file.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg

@@ -0,0 +1,58 @@
+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.