linux-kernel - Re: [PATCH v4 12/17] drm/uAPI: Add "preferred color format" drm property as setting for userspace

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Date:   Tue, 29 Jun 2021 08:12:54 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Werner Sembach <wse@...edocomputers.com>,
        amd-gfx@...ts.freedesktop.org, tzimmermann@...e.de,
        intel-gfx@...ts.freedesktop.org, sunpeng.li@....com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        airlied@...ux.ie, rodrigo.vivi@...el.com,
        alexander.deucher@....com, christian.koenig@....com
Subject: Re: [PATCH v4 12/17] drm/uAPI: Add "preferred color format" drm property as setting for userspace

On Tuesday, June 22nd, 2021 at 09:15, Pekka Paalanen <ppaalanen@...il.com> wrote:

> yes, I think this makes sense, even if it is a property that one can't
> tell for sure what it does before hand.
>
> Using a pair of properties, preference and active, to ask for something
> and then check what actually worked is good for reducing the
> combinatorial explosion caused by needing to "atomic TEST_ONLY commit"
> test different KMS configurations. Userspace has a better chance of
> finding a configuration that is possible.
>
> OTOH, this has the problem than in UI one cannot tell the user in
> advance which options are truly possible. Given that KMS properties are
> rarely completely independent, and in this case known to depend on
> several other KMS properties, I think it is good enough to know after
> the fact.
>
> If a driver does not use what userspace prefers, there is no way to
> understand why, or what else to change to make it happen. That problem
> exists anyway, because TEST_ONLY commits do not give useful feedback
> but only a yes/no.

By submitting incremental atomic reqs with TEST_ONLY (i.e. only changing one
property at a time), user-space can discover which property makes the atomic
commit fail.

I'm not a fan of this "preference" property approach. The only way to find out
whether it's possible to change the color format is to perform a user-visible
change (with a regular atomic commit) and check whether it worked
after-the-fact. This is unlike all other existing KMS properties.

I'd much rather see a more general approach to fix this combinatorial explosion
than to add special-cases like this.