On Mon, 13 Nov 2023 09:18:39 +0000 Simon Ser <cont...@emersion.fr> wrote:
> On Monday, October 23rd, 2023 at 10:25, Simon Ser <cont...@emersion.fr> wrote: > > > > > > > > > > > > +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. [...] > > > > > > > > > > > During the hackfest in Brno, it was mentioned that a > > > > > > > > > > > commit which re-sets the same FB_ID could actually have > > > > > > > > > > > an effect with VRR: It could trigger scanout of the next > > > > > > > > > > > frame before vertical blank has reached its maximum > > > > > > > > > > > duration. Some kind of mechanism is required for this in > > > > > > > > > > > order to allow user space to perform low frame rate > > > > > > > > > > > compensation. > > > > > > > > > > > > > > > > > > Xaver tested this hypothesis in a flipping the same fb in a > > > > > > > > > VRR monitor > > > > > > > > > and it worked as expected, so this shouldn't be a concern. > > > > > > > > > Right, so it must have some effect. It cannot be simply > > > > > > > > > ignored like in > > > > > > > > > the proposed doc wording. Do we special-case re-setting the > > > > > > > > > same FB_ID > > > > > > > > > as "not a no-op" or "not ignored" or some other way? > > > > > > > > > There's an effect in the refresh rate, the image won't change > > > > > > > > > but it > > > > > > > > > will report that a flip had happened asynchronously so the > > > > > > > > > reported > > > > > > > > > framerate will be increased. Maybe an additional wording > > > > > > > > > could be like: > > > > > > > > > > > > > > Flipping to the same FB_ID will result in a immediate flip as if > > > > > > > it was > > > > > > > changing to a different one, with no effect on the image but > > > > > > > effecting > > > > > > > the reported frame rate. > > > > > > > > > > > > Re-setting FB_ID to its current value is a special case regardless > > > > > > of > > > > > > PAGE_FLIP_ASYNC, is it not? > > > > > > > > > > No. The rule has so far been that all side effects are observed > > > > > even if you flip to the same fb. And that is one of my annoyances > > > > > with this proposal. The rules will now be different for async flips > > > > > vs. everything else. > > > > > > > > Well with the patches the async page-flip case is exactly the same as > > > > the non-async page-flip case. In both cases, if a FB_ID is included in > > > > an atomic commit then the side effects are triggered even if the > > > > property > > > > value didn't change. The rules are the same for everything. > > > > > > I see it only checking if FB_ID changes or not. If it doesn't > > > change then the implication is that the side effects will in > > > fact be skipped as not all planes may even support async flips. > > > > Hm right. So the problem is that setting any prop = same value as > > previous one will result in a new page-flip for asynchronous page-flips, > > but will not result in any side-effect for asynchronous page-flips. > > > > Does it actually matter though? For async page-flips, I don't think this > > would result in any actual difference in behavior? Hi Simon, a fly-by question... > To sum this up, here is a matrix of behavior as seen by user-space: > > - Sync atomic page-flip > - Set FB_ID to different value: programs hw for page-flip, sends uevent > - Set FB_ID to same value: same (important for VRR) > - Set another plane prop to same value: same > - Set another plane prop to different value: maybe rejected if modeset > required > - Async atomic page-flip > - Set FB_ID to different value: updates hw with new FB address, sends > immediate uevent > - Set FB_ID to same value: same (no-op for the hw) It should not be a no-op for the hw, because the hw might be in the middle of a VRR front-porch waiting period, and the commit needs to end the waiting immediately rather than time out? > - Set another plane prop to same value: ignored, sends immediate uevent > (special codepath) If the sync case says "same", then shouldn't this say "same" as well to be consistent? > - Set another plane prop to different value: always rejected > > To me sync and async look consistent. Thanks, pq
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