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

[Johannes Berg]

On Thu, 2022-07-21 at 14:43 +1000, Dave Airlie wrote:
> 
> +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.

That seems sane, and certainly something we've done in wireless in the
past.

> +* 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,

Eh, I went to write a whole paragraph here and then read it again ...
Maybe this should say "[t]he _full_ firmware version", to contrast with
the previous sentence mentioning the "major version".

>  in order to let the driver detact any non-ABI

typo - 'detect'

> +  fixes/changes. The firmware files in linux-firmware should be
> +  overwritten with the newest compatible major version.
> 

That's also a bit confusing IMHO - did that mean "minor version"? Or
something? I mean ... if you overwrite a file that has the major version
in the filename then by definition it is the same major version?

> +  This means no major version bumps without the kernel retaining
> +  backwards compatibility for the older major versions.

Strictly reading this might require aeons of support for firmware
version, if you have a release cadence of them like every 6 weeks for a
new _major_ version (yes, because APIs change), then that's rather
harsh. In practice we've often done this, but I think some reasonable
cut-off could/should be there, such as dropping support after a
reasonably long time frame (say a year?)

Often though that's less a question of "does it still work" and rather
one of "do I still support that" and the answer for the latter is
obviously "no" much quicker than the former.

johannes