docs: driver-api: firmware: add driver firmware guidelines. (v3)

A recent snafu where Intel ignored upstream feedback on a firmware
change, led to a late rc6 fix being required. In order to avoid this
in the future we should document some expectations around
linux-firmware.

I was originally going to write this for drm, but it seems quite generic
advice.

v2: rewritten with suggestions from Thorsten Leemhuis
v3: rewritten with suggestions from Mauro
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://lore.kernel.org/r/20220721044352.3110507-1-airlied@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/firmware/core.rst

@@ -13,4 +13,5 @@ documents these features.
    direct-fs-lookup
    fallback-mechanisms
    lookup-order
+   firmware-usage-guidelines
 

Documentation/driver-api/firmware/firmware-usage-guidelines.rst

+===================
+Firmware Guidelines
+===================
+
+Users switching to a newer kernel should *not* have to install newer
+firmware files to keep their hardware working. At the same time updated
+firmware files must not cause any regressions for users of older kernel
+releases.
+
+Drivers that use firmware from linux-firmware should follow the rules in
+this guide. (Where there is limited control of the firmware,
+i.e. company doesn't support Linux, firmwares sourced from misc places,
+then of course these rules will not apply strictly.)
+
+* Firmware files shall be designed in a way that it allows checking for
+  firmware ABI version changes. It is recommended that firmware files be
+  versioned with at least a major/minor version. It is suggested that
+  the firmware files in linux-firmware be named with some device
+  specific name, and just the major version. The firmware version should
+  be stored in the firmware header, or as an exception, as part of the
+  firmware file name, in order to let the driver detact any non-ABI
+  fixes/changes. The firmware files in linux-firmware should be
+  overwritten with the newest compatible major version. Newer major
+  version firmware shall remain compatible with all kernels that load
+  that major number.
+
+* If the kernel support for the hardware is normally inactive, or the
+  hardware isn't available for public consumption, this can
+  be ignored, until the first kernel release that enables that hardware.
+  This means no major version bumps without the kernel retaining
+  backwards compatibility for the older major versions.  Minor version
+  bumps should not introduce new features that newer kernels depend on
+  non-optionally.
+
+* If a security fix needs lockstep firmware and kernel fixes in order to
+  be successful, then all supported major versions in the linux-firmware
+  repo that are required by currently supported stable/LTS kernels,
+  should be updated with the security fix. The kernel patches should
+  detect if the firmware is new enough to declare if the security issue
+  is fixed.  All communications around security fixes should point at
+  both the firmware and kernel fixes. If a security fix requires
+  deprecating old major versions, then this should only be done as a
+  last option, and be stated clearly in all communications.
+