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Subject: [PATCH v7 5/6] drm/doc: Define KMS atomic state set
Date:   Tue, 17 Oct 2023 11:28:36 +0200
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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: André Almeida <andrealmeid@igalia.com>
---
v7:
- add a note that drivers can make exceptions for ad-hoc prop changes
- add a note about flipping the same FB_ID as a no-op
---
 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 632989df3727..34bd02270ee7 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.
-- 
2.42.0