[Bug 101506] [HSW] desktop gets laggy while prime offloading

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101506

--- Comment #1 from Chris Wilson <chris at chris-wilson.co.uk> ---
The patch I gave you would have skipped the sync on the nouveau fence.

There's something not quite right here. The desktop shouldn't be capped to the
throughput on the nouveau device ... unless it too was rendering on the dGPU,
or the buffer exchange is not quite right (and we are recycling buffers too
early and causing a fence cycle). It would stutter when syncing on new frames
from the dGPU, but in between it would use the current content to re-render.

One solution is to only queue the copy from dGPU->iGPU once the dGPU is
complete. Present does provide a fence to delay the PresentPixmap which mesa
could fill in to tell Present to wait until it has completed the dGPU task.
That would probably require handing off to a thread so that it could wait on
the dGPU fence before signaling the X fence.

dri2 could be taught a similar trick, but simpler since we can just defer the
task inside X until the next vblank.

The complication really is deducing where in the chain of client / X /
compositor / X do we need to apply the decoupling, i.e. who do we fix. In the
wayland world, it is client / compositor and it is much more obvious about how
to organise the compositor to avoid the implicit sync.

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