linux-kernel - Re: [PATCH] drm/doc: Define KMS atomic state set

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Date:   Fri, 1 Dec 2023 09:29:05 +0100
From:   Maxime Ripard <mripard@...nel.org>
To:     André Almeida <andrealmeid@...lia.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel-dev@...lia.com, alexander.deucher@....com,
        christian.koenig@....com, Simon Ser <contact@...rsion.fr>,
        Rob Clark <robdclark@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>, daniel@...ll.ch,
        Daniel Stone <daniel@...ishbar.org>,
        'Marek Olšák' <maraeo@...il.com>,
        Dave Airlie <airlied@...il.com>,
        Michel Dänzer <michel.daenzer@...lbox.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Pekka Paalanen <pekka.paalanen@...labora.com>
Subject: Re: [PATCH] drm/doc: Define KMS atomic state set

Hi,

On Thu, Nov 30, 2023 at 05:07:40PM -0300, André Almeida wrote:
> From: Pekka Paalanen <pekka.paalanen@...labora.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@...labora.com>
> Signed-off-by: André Almeida <andrealmeid@...lia.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

That part is pretty confusing to me.

What are you calling the current and old KMS state?

What's confusing to me is that, yes, what you're saying is true for a
given object: if it was part of the commit, the new state is the old
state + whatever the new state changed.

However, if that object wasn't part of the commit at all, then it's
completely out of the old or new global KMS state.

So yeah, individual object KMS state are indeed complete, but
drm_atomic_state definitely isn't. And it's the whole point of functions
like drm_atomic_get_crtc_state() vs drm_atomic_get_old/new_crtc_state:
the old/new variants only return a state if it was part of
drm_atomic_state to begin with. drm_atomic_get_crtc_state() brings the
crtc state into drm_atomic_state if it wasn't part of it.

Maxime

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