drm/compositor: option to keep previous frame's primary buffer alive

[aoli-al]

Description

Introduce an option to enable 3 buffer rotation. Currently, Swapchain::acquire returns the first free slot, which is nearlly always the most recent released buffer. The nvidia driver appears to keep reading the buffer for a short while. Holding the buffer for 1 frame longer will avoid flickering.

This matches the behavior in Hyprland, which does not exhibit flickering.

This fixes: #1862

I use claude code + fable to understand the implementation differences between smithay and hyprland and draft the PR

Checklist

• I agree to smithay's Developer Certificate of Origin.

[Drakulix]

Thanks for debugging this, but that seems like a more problematic synchronization issue, that I wouldn't want to solely solve with a workaround in DrmCompositor.

@cmeissl Maybe we should set OUT_FENCE_PTR and attach that as a sort of "blocker" to the buffer? I would expect the DRM_EVENT_FLIP_COMPLETE to be enough, though it is actually hard to figure out what guarantees that holds.

[aoli-al]

Thanks! Yes, I agree the current solution is a bit unfortunate. This is definitely a driver bug but it seems that Nvidia is not doing anything 🙁

Happy to test on my end if you have other solutions.