From: "Gabriel Somlo" <somlo@cmu.edu>

Allow access to QEMU firmware blobs, passed into the guest VM via
the fw_cfg device, through SysFS entries. Blob meta-data (e.g. name,
size, and fw_cfg key), as well as the raw binary blob data may be
accessed.

The SysFS access location is /sys/firmware/qemu_fw_cfg/... and was
selected based on overall similarity to the type of information
exposed under /sys/firmware/dmi/entries/...

This functionality is primarily intended to serve as a host->guest
configuration data transfer mechanism that is both:

        - asynchronous: the host doesn't need to wait for the guest
                        to be ready to accept data (e.g., by starting
                        an agent daemon)

        - out-of-band:  there is no need to commandeer a guest element
                        normally visible and available to the guest user
                        (e.g., kernel cmdline, mounting floppy/cdrom, etc.)

QEMU now allows arbitrary fw_cfg blobs to be added via the command line,
so it would be nice to make them easy to retrieve from within the guest
OS, and the niceset and easiest way I can think of is

        cat /sys/firmware/qemu-fw-cfg/.../<some-blob-name>/raw

New since v7:

	- kbuild testbot complained about a warning on ia64, so limit
	  the availability of fw_cfg in Kconfig to only architectures
	  on which QEMU makes it available, which include:

		(ARM || ARM64 || PPC_PMAC || SPARC || X86)

Thanks,
  --Gabriel

New since v6:

- added architecture-specific default values for fw_cfg register
  offsets: DT and/or ACPI will only give us the base address and
  total size of the fw_cfg register set, but not individual register
  offsets *within* this total extent. The specific offsets are
  different across architectures, and this version adds #defines
  so that reasonable defaults can be used on each supported platform.

quoted

New since v5:

- fixed typos in documentation files (Patches 1/4 and 4/4

- printf/scanf type modifier for phys_addr_t now matches
  arch-specific width (u32 vs. u64), avoiding compiler warnings.
  (tested on i386 with and without PAE, and on armv7hl with and
   without lpae -- the latter pair took quite a while on an
   emulated QEMU guest :) )

quoted

New since v4:

Documentation (Patches 1/4 and 4/4) now points to the authoritative
file in the QEMU source tree for any details related to the "hardware
interface" of the fw_cfg device; Only details specific to sysfs (1/4) 
and DT (4/4) should stay in the kernel docs.

quoted

New (since v3):

Patch 1/4: Device probing now works with either ACPI, DT, or
	   optionally by manually specifying a base, size, and
	   register offsets on the command line. This way, all
	   architectures offering fw_cfg can be supported, although
	   x86 and ARM get *automatic* support via ACPI and/or DT.

	   HUGE thanks to Laszlo Ersek [off-list ref] for
	   pointing out drivers/virtio/virtio_mmio.c, as an example
	   on how to pull this off !!!

	   Stefan: I saw Marc's DMA patches to fw_cfg. Since only
	   x86 and ARM will support it starting with QEMU 2.5, and
	   since I expect to get lots of otherwise interesting (but
	   otherwise orthogonal) feedback on this series, I'd like
	   to stick with ioread8() across the board for now. We can
	   always patch in DMA support in a backward compatible way
	   later, once this series gets (hopefully) accepted :)

Patch 2/4: (was 3/4 in v3): unchanged. Exports kset_find_obj() so
	   modules can call it.

Patch 3/4: (was 4/4 in v3): rebased, but otherwise the same.
	   Essentially, creates a "human readable" directory
	   hierarchy from "path-like" tokens making up fw_cfg
	   blob names. I'm not really sure there's a way to make
	   this happen via udev rules, but I have at least one
	   potential use case for doing it *before* udev becomes
	   available (cc: Andy Lutomirski [off-list ref]),
	   so I'd be happy to leave this functionality in the
	   kernel module. See further below for an illustration
	   of this.

Patch 4/4: Updates the existing ARM DT documentation for fw_cfg,
	   mainly by pointing at the more comprehensive document
	   introduced with Patch 1/4 for details on the fw_cfg
	   device interface, leaving only the specific ARM/DT
	   address/size node information in place.

quoted

 In addition to the "by_key" blob listing, e.g.:
 
 $ tree /sys/firmware/qemu_fw_cfg/
 /sys/firmware/qemu_fw_cfg/
 |-- by_key
 |   |-- 32
 |   |   |-- key
 |   |   |-- name                        ("etc/boot-fail-wait")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 33
 |   |   |-- key
 |   |   |-- name                        ("etc/smbios/smbios-tables")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 34
 |   |   |-- key
 |   |   |-- name                        ("etc/smbios/smbios-anchor")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 35
 |   |   |-- key
 |   |   |-- name                        ("etc/e820")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 36
 |   |   |-- key
 |   |   |-- name                        ("genroms/kvmvapic.bin")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 37
 |   |   |-- key
 |   |   |-- name                        ("etc/system-states")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 38
 |   |   |-- key
 |   |   |-- name                        ("etc/acpi/tables")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 39
 |   |   |-- key
 |   |   |-- name                        ("etc/table-loader")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 40
 |   |   |-- key
 |   |   |-- name                        ("etc/tpm/log")
 |   |   |-- raw
 |   |   `-- size
 |   |-- 41
 |   |   |-- key
 |   |   |-- name                        ("etc/acpi/rsdp")
 |   |   |-- raw
 |   |   `-- size
 |   `-- 42
 |       |-- key
 |       |-- name                        ("bootorder")
 |       |-- raw
 |       `-- size
 |
 ...
 
 Patch 3/4 also gets us a "human readable" "by_name" listing, like so:
 
 ...
 |-- by_name
 |   |-- bootorder -> ../by_key/42
 |   |-- etc
 |   |   |-- acpi
 |   |   |   |-- rsdp -> ../../../by_key/41
 |   |   |   `-- tables -> ../../../by_key/38
 |   |   |-- boot-fail-wait -> ../../by_key/32
 |   |   |-- e820 -> ../../by_key/35
 |   |   |-- smbios
 |   |   |   |-- smbios-anchor -> ../../../by_key/34
 |   |   |   `-- smbios-tables -> ../../../by_key/33
 |   |   |-- system-states -> ../../by_key/37
 |   |   |-- table-loader -> ../../by_key/39
 |   |   `-- tpm
 |   |       `-- log -> ../../../by_key/40
 |   `-- genroms
 |       `-- kvmvapic.bin -> ../../by_key/36
 `-- rev

Gabriel Somlo (4):
  firmware: introduce sysfs driver for QEMU's fw_cfg device
  kobject: export kset_find_obj() for module use
  firmware: create directory hierarchy for sysfs fw_cfg entries
  devicetree: update documentation for fw_cfg ARM bindings

 .../ABI/testing/sysfs-firmware-qemu_fw_cfg         | 100 +++
 Documentation/devicetree/bindings/arm/fw-cfg.txt   |  38 +-
 drivers/firmware/Kconfig                           |  19 +
 drivers/firmware/Makefile                          |   1 +
 drivers/firmware/qemu_fw_cfg.c                     | 751 +++++++++++++++++++++
 lib/kobject.c                                      |   1 +
 6 files changed, 874 insertions(+), 36 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
 create mode 100644 drivers/firmware/qemu_fw_cfg.c

-- 
2.4.3