On Tue, 11 Jul 2023 10:57:57 +0200 Daniel Vetter <dan...@ffwll.ch> wrote:
> On Fri, Jul 07, 2023 at 07:40:59PM -0300, André Almeida wrote: > > From: Pekka Paalanen <pekka.paala...@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.paala...@collabora.com> > > Signed-off-by: André Almeida <andrealm...@igalia.com> > > --- > > 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.vet...@ffwll.ch> After talking on IRC yesterday, we realized that the no-op rule is nowhere near as generic as I have believed. Roughly: https://oftc.irclog.whitequark.org/dri-devel/2023-07-12#1689152446-1689157291; Thanks, pq > > + > > +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 > > >
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