[PATCH] drm/doc: Define KMS atomic state set

[Bagas Sanjaya]

On Thu, Nov 30, 2023 at 05:07:40PM -0300, André Almeida wrote:
> From: Pekka Paalanen <pekka.paalanen at 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 at collabora.com>
> Signed-off-by: André Almeida <andrealmeid at igalia.com>
> ---
> 
> 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.com/
> 
>  Documentation/gpu/drm-uapi.rst | 47 ++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
> 
> 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
>  
>  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
> +================
> +
> +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 is 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 unexpectedly
> +fail, cause visible glitches, or delay reaching the final state.
> +
> +An atomic commit can be flagged with DRM_MODE_ATOMIC_TEST_ONLY, which means the
> +complete state change is validated but not applied.  Userspace should use this
> +flag to validate any state change before asking to apply it. If validation fails
> +for any reason, userspace should attempt to fall back to another, perhaps
> +simpler, final state.  This allows userspace to probe for various configurations
> +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 state in
> +the kernel. Hence, the complete new KMS state is the complete old KMS state 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 property
> +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 that some
> +redundant information might result in oversynchronization issues.  No-operation
> +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 not be a
> +modeset on its own. As a special exception for VRR needs, explicitly setting
> +FB_ID to its current value is not a no-op.
> +
> +A "modeset" is a change in KMS state that might enable, disable, or temporarily
> +disrupt the emitted video signal, possibly causing visible glitches on screen. A
> +modeset may also take considerably more time to complete than other kinds of
> +changes, and the video sink might also need time to adapt to the new signal
> +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 change 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 changes
> +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 at gmail.com>

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