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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>
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