firmware: google: Implement cbmem in sysfs driver

The CBMEM area is a downward-growing memory region used by coreboot to
dynamically allocate tagged data structures ("CBMEM entries") that
remain resident during boot.

This implements a driver which exports access to the CBMEM entries
via sysfs under /sys/bus/coreboot/devices/cbmem-<id>.

This implementation is quite versatile.  Examples of how it could be
used are given below:

* Tools like util/cbmem from the coreboot tree could use this driver
  instead of finding CBMEM in /dev/mem directly.  Alternatively,
  firmware developers debugging an issue may find the sysfs interface
  more ergonomic than the cbmem tool and choose to use it directly.

* The crossystem tool, which exposes verified boot variables, can use
  this driver to read the vboot work buffer.

* Tools which read the BIOS SPI flash (e.g., flashrom) can find the
  flash layout in CBMEM directly, which is significantly faster than
  searching the flash directly.

Write access is provided to all CBMEM regions via
/sys/bus/coreboot/devices/cbmem-<id>/mem, as the existing cbmem
tooling updates this memory region, and envisioned use cases with
crossystem can benefit from updating memory regions.

Link: https://issuetracker.google.com/239604743
Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>
Link: https://lore.kernel.org/r/20221104161528.531248-1-jrosenth@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 45 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-coreboot b/Documentation/ABI/testing/sysfs-bus-coreboot
new file mode 100644
index 000000000000..9c5accecc470
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-coreboot
@@ -0,0 +1,45 @@
+What:		/sys/bus/coreboot
+Date:		August 2022
+Contact:	Jack Rosenthal <jrosenth@chromium.org>
+Description:
+		The coreboot bus provides a variety of virtual devices used to
+		access data structures created by the Coreboot BIOS.
+
+What:		/sys/bus/coreboot/devices/cbmem-<id>
+Date:		August 2022
+Contact:	Jack Rosenthal <jrosenth@chromium.org>
+Description:
+		CBMEM is a downwards-growing memory region created by Coreboot,
+		and contains tagged data structures to be shared with payloads
+		in the boot process and the OS.  Each CBMEM entry is given a
+		directory in /sys/bus/coreboot/devices based on its id.
+		A list of ids known to Coreboot can be found in the coreboot
+		source tree at
+		``src/commonlib/bsd/include/commonlib/bsd/cbmem_id.h``.
+
+What:		/sys/bus/coreboot/devices/cbmem-<id>/address
+Date:		August 2022
+Contact:	Jack Rosenthal <jrosenth@chromium.org>
+Description:
+		This is the pyhsical memory address that the CBMEM entry's data
+		begins at, in hexadecimal (e.g., ``0x76ffe000``).
+
+What:		/sys/bus/coreboot/devices/cbmem-<id>/size
+Date:		August 2022
+Contact:	Jack Rosenthal <jrosenth@chromium.org>
+Description:
+		This is the size of the CBMEM entry's data, in hexadecimal
+		(e.g., ``0x1234``).
+
+What:		/sys/bus/coreboot/devices/cbmem-<id>/mem
+Date:		August 2022
+Contact:	Jack Rosenthal <jrosenth@chromium.org>
+Description:
+		A file exposing read/write access to the entry's data.  Note
+		that this file does not support mmap(), as coreboot
+		does not guarantee that the data will be page-aligned.
+
+		The mode of this file is 0600.  While there shouldn't be
+		anything security-sensitive contained in CBMEM, read access
+		requires root privileges given this is exposing a small subset
+		of physical memory.