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Date:   Fri, 1 Dec 2023 08:07:40 +0700
From:   Bagas Sanjaya <bagasdotme@gmail.com>
To:     =?utf-8?B?QW5kcsOp?= Almeida <andrealmeid@igalia.com>,
        Linux DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Cc:     kernel-dev@igalia.com, alexander.deucher@amd.com,
        christian.koenig@amd.com, Simon Ser <contact@emersion.fr>,
        Rob Clark <robdclark@gmail.com>,
        Pekka Paalanen <ppaalanen@gmail.com>, daniel@ffwll.ch,
        Daniel Stone <daniel@fooishbar.org>,
        'Marek =?utf-8?B?T2zFocOhayc=?= <maraeo@gmail.com>,
        Dave Airlie <airlied@gmail.com>,
        Michel =?utf-8?Q?D=C3=A4nzer?= <michel.daenzer@mailbox.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Maxime Ripard <mripard@kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Pekka Paalanen <pekka.paalanen@collabora.com>,
        Attreyee M <tintinm2017@gmail.com>
Subject: Re: [PATCH] drm/doc: Define KMS atomic state set
Message-ID: <ZWkx3IcUTxO-IdIK@archie.me>
References: <20231130200740.53454-1-andrealmeid@igalia.com>
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On Thu, Nov 30, 2023 at 05:07:40PM -0300, Andr=C3=A9 Almeida wrote:
> From: Pekka Paalanen <pekka.paalanen@collabora.com>
>=20
> Specify how the atomic state is maintained between userspace and
> kernel, plus the special case for async flips.
>=20
> Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> Signed-off-by: Andr=C3=A9 Almeida <andrealmeid@igalia.com>
> ---
>=20
> This is a standalone patch from the following serie, the other patches are
> already merged:
> https://lore.kernel.org/lkml/20231122161941.320564-1-andrealmeid@igalia.c=
om/
>=20
>  Documentation/gpu/drm-uapi.rst | 47 ++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
>=20
> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.=
rst
> index 370d820be248..d0693f902a5c 100644
> --- a/Documentation/gpu/drm-uapi.rst
> +++ b/Documentation/gpu/drm-uapi.rst
> @@ -570,3 +570,50 @@ dma-buf interoperability
> =20
>  Please see Documentation/userspace-api/dma-buf-alloc-exchange.rst for
>  information on how dma-buf is integrated and exposed within DRM.
> +
> +KMS atomic state
> +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> +
> +An atomic commit can change multiple KMS properties in an atomic fashion,
> +without ever applying intermediate or partial state changes.  Either the=
 whole
> +commit succeeds or fails, and it will never be applied partially. This i=
s the
> +fundamental improvement of the atomic API over the older non-atomic API =
which is
> +referred to as the "legacy API".  Applying intermediate state could unex=
pectedly
> +fail, cause visible glitches, or delay reaching the final state.
> +
> +An atomic commit can be flagged with DRM_MODE_ATOMIC_TEST_ONLY, which me=
ans the
> +complete state change is validated but not applied.  Userspace should us=
e this
> +flag to validate any state change before asking to apply it. If validati=
on fails
> +for any reason, userspace should attempt to fall back to another, perhaps
> +simpler, final state.  This allows userspace to probe for various config=
urations
> +without causing visible glitches on screen and without the need to undo a
> +probing change.
> +
> +The changes recorded in an atomic commit apply on top the current KMS st=
ate in
> +the kernel. Hence, the complete new KMS state is the complete old KMS st=
ate with
> +the committed property settings done on top. The kernel will try to avoid
> +no-operation changes, so it is safe for userspace to send redundant prop=
erty
> +settings.  However, not every situation allows for no-op changes, due to=
 the
> +need to acquire locks for some attributes. Userspace needs to be aware t=
hat some
> +redundant information might result in oversynchronization issues.  No-op=
eration
> +changes do not count towards actually needed changes, e.g.  setting MODE=
_ID to a
> +different blob with identical contents as the current KMS state shall no=
t be a
> +modeset on its own. As a special exception for VRR needs, explicitly set=
ting
> +FB_ID to its current value is not a no-op.
> +
> +A "modeset" is a change in KMS state that might enable, disable, or temp=
orarily
> +disrupt the emitted video signal, possibly causing visible glitches on s=
creen. A
> +modeset may also take considerably more time to complete than other kind=
s of
> +changes, and the video sink might also need time to adapt to the new sig=
nal
> +properties. Therefore a modeset must be explicitly allowed with the flag
> +DRM_MODE_ATOMIC_ALLOW_MODESET.  This in combination with
> +DRM_MODE_ATOMIC_TEST_ONLY allows userspace to determine if a state chang=
e is
> +likely to cause visible disruption on screen and avoid such changes when=
 end
> +users do not expect them.
> +
> +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-operation c=
hanges
> +are ignored as always. Changing any other property will cause the commit=
 to be
> +rejected. Each driver may relax this restriction if they have guarantees=
 that
> +such property change doesn't cause modesets. Userspace can use TEST_ONLY=
 commits
> +to query the driver about this.

The wording LGTM, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>

--=20
An old man doll... just what I always wanted! - Clara

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