Received-SPF: pass (google.com: domain of linux-wireless-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <4f8ab262d98ba2a4d0e106e127c171e75b52ad47.camel@sipsolutions.net>
Subject: Re: [PATCH] docs: driver-api: firmware: add driver firmware
 guidelines. (v3)
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Dave Airlie <airlied@gmail.com>, torvalds@linux-foundation.org,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        gregkh@linuxfoundation.org, Daniel Vetter <daniel@ffwll.ch>,
        mcgrof@kernel.org
Cc:     linux-kernel@vger.kernel.org, dri-devel@lists.sf.net,
        netdev@vger.kernel.org, linux-wireless@vger.kernel.org,
        alsa-devel@alsa-project.org, linux-media@vger.kernel.org,
        linux-block@vger.kernel.org, Dave Airlie <airlied@redhat.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Harry Wentland <harry.wentland@amd.com>
Date:   Sun, 24 Jul 2022 17:21:43 +0200
In-Reply-To: <20220721044352.3110507-1-airlied@gmail.com>
References: <20220721044352.3110507-1-airlied@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.44.3 (3.44.3-1.fc36) 
MIME-Version: 1.0
Precedence: bulk

On Thu, 2022-07-21 at 14:43 +1000, Dave Airlie wrote:
>=20
> +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.
>=20

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