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Date:   Mon, 16 Oct 2023 15:18:56 +0300
From:   Pekka Paalanen <ppaalanen@gmail.com>
To:     =?UTF-8?B?QW5kcsOp?= Almeida <andrealmeid@igalia.com>
Cc:     Michel =?UTF-8?B?RMOkbnplcg==?= <michel.daenzer@mailbox.org>,
        xaver.hugl@gmail.com, pierre-eric.pelloux-prayer@amd.com,
        Daniel Vetter <daniel@ffwll.ch>,
        'Marek =?UTF-8?B?T2zFocOhayc=?= <maraeo@gmail.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Daniel Stone <daniel@fooishbar.org>,
        linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org,
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        Rob Clark <robdclark@gmail.com>, ville.syrjala@linux.intel.com,
        kernel-dev@igalia.com, alexander.deucher@amd.com,
        Dave Airlie <airlied@gmail.com>, christian.koenig@amd.com,
        joshua@froggi.es, wayland-devel@lists.freedesktop.org
Subject: Re: [PATCH v6 6/6] drm/doc: Define KMS atomic state set
Message-ID: <20231016151856.74af9305@eldfell>
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On Mon, 16 Oct 2023 12:52:32 +0200
Andr=C3=A9 Almeida <andrealmeid@igalia.com> wrote:

> Hi Michel,
>=20
> On 8/17/23 12:37, Michel D=C3=A4nzer wrote:
> > On 8/15/23 20:57, Andr=C3=A9 Almeida wrote: =20
> >> From: Pekka Paalanen <pekka.paalanen@collabora.com>
> >>
> >> Specify how the atomic state is maintained between userspace and
> >> kernel, plus the special case for async flips.
> >>
> >> Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> >> Signed-off-by: Andr=C3=A9 Almeida <andrealmeid@igalia.com> =20
> > [...]
> > =20
> >> +An atomic commit with the flag DRM_MODE_PAGE_FLIP_ASYNC is allowed to
> >> +effectively change only the FB_ID property on any planes. No-operatio=
n changes
> >> +are ignored as always. [...] =20
> > During the hackfest in Brno, it was mentioned that a commit which re-se=
ts the same FB_ID could actually have an effect with VRR: It could trigger =
scanout of the next frame before vertical blank has reached its maximum dur=
ation. Some kind of mechanism is required for this in order to allow user s=
pace to perform low frame rate compensation.
> > =20
> Xaver tested this hypothesis in a flipping the same fb in a VRR monitor=20
> and it worked as expected, so this shouldn't be a concern.

Right, so it must have some effect. It cannot be simply ignored like in
the proposed doc wording. Do we special-case re-setting the same FB_ID
as "not a no-op" or "not ignored" or some other way?


Thanks,
pq

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