On Fri, Jul 07, 2023 at 07:40:59PM -0300, André Almeida wrote: > From: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxx> > Signed-off-by: André Almeida <andrealmeid@xxxxxxxxxx> > --- > v4: total rework by Pekka > --- > Documentation/gpu/drm-uapi.rst | 41 ++++++++++++++++++++++++++++++++++ > 1 file changed, 41 insertions(+) > > diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst > index 65fb3036a580..6a1662c08901 100644 > --- a/Documentation/gpu/drm-uapi.rst > +++ b/Documentation/gpu/drm-uapi.rst > @@ -486,3 +486,44 @@ and the CRTC index is its position in this array. > > .. kernel-doc:: include/uapi/drm/drm_mode.h > :internal: > + > +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 automatically avoid > +no-operation changes, so it is safe and even expected for userspace to send > +redundant property settings. 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. Small clarification: The kernel indeed tries very hard to make redundant changes a no-op, and I think we should consider any issues here bugs. But it still has to check, which means it needs to acquire the right locks and put in the right (cross-crtc) synchronization points, and due to implmentation challenges it's very hard to try to avoid that in all cases. So adding redundant changes especially across crtc (and their connected planes/connectors) might result in some oversynchronization issues, and userspace should therefore avoid them if feasible. With some sentences added to clarify this: Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx> > + > +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. > -- > 2.41.0 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
