Implement wayland side of tearing control

[PolyMeilex]

Probably needs some work on DRM side, but this does plumbing for the wayland side.

[Drakulix]

Wayland side LGTM.

I think all this need for drm is:

• Check for the ASyncPageFlip capability (https://docs.rs/drm/latest/drm/enum.DriverCapability.html#variant.ASyncPageFlip) - This has to be done per-device, so this can't really affect the global being advertised.
• Set PAGE_FLIP_ASYNC on page_flip: https://docs.rs/drm/latest/drm/control/struct.AtomicCommitFlags.html#associatedconstant.PAGE_FLIP_ASYNC

The latter should only happen, when a surface with that hint set is fullscreen though. Possibly the DrmCompositor could extract that state from the surface render-elements (might need a new method for RenderElement and apply that automatically, when directly scanning out such a surface on the primary plane. (cc @cmeissl)

[cmeissl]

The latter should only happen, when a surface with that hint set is fullscreen though. Possibly the DrmCompositor could extract that state from the surface render-elements (might need a new method for RenderElement and apply that automatically, when directly scanning out such a surface on the primary plane. (cc @cmeissl)

I see a few different ways to implement that:

1. Semi-automatically in the DrmCompositor, which like you said requires extending the trait by a function informing us about the preference. I believe we might be able to make the detection a bit more intelligent then basing it off primary plane scan-out. For example it might be ok to tear if only a single surface (that has tearing enabled) on an overlay plane is updated and everything else stays the same. Or a bit simpler and probably good enough, if all elements are directly scanned out and allow tearing.
2. As a parameter/option of DrmCompositor and allow downstream decide based on whatever
3. Combination of 1. and 2., maybe some option on DrmCompositor which allows to select Auto, Disabled, Force or something like this.

Option 2. and 3. would allow to expose some global preference, like disabling tearing even if it would be possible.

[YaLTeR]

I agree with @cmeissl; I'd like more manual tearing control in my compositor. So that I can implement the fullscreen with hint => tear policy, but also other policies, like maybe I want to enable it for windowed too under some conditions.

[PolyMeilex]

I agree with @cmeissl; I'd like more manual tearing control in my compositor. So that I can implement the fullscreen with hint => tear policy, but also other policies, like maybe I want to enable it for windowed too under some conditions.

For now I'm working on a simplest possible setup, you either queue a frame as VSync or ASync, you can see it or even test it here
My kernel does not support Atomic async page flips yet, so I have not yet tested that code path, but legacy one with SMITHAY_USE_LEGACY env variable works already on my system.