[Mesa-dev] Plumbing explicit synchronization through the Linux ecosystem

[Lucas Stach]

Am Dienstag, den 17.03.2020, 10:12 -0700 schrieb Jacob Lifshay:
> One related issue with explicit sync using sync_file is that combined
> CPUs/GPUs (the CPU cores *are* the GPU cores) that do all the
> rendering in userspace (like llvmpipe but for Vulkan and with extra
> instructions for GPU tasks) but need to synchronize with other
> drivers/processes is that there should be some way to create an
> explicit fence/semaphore from userspace and later signal it. This
> seems to conflict with the requirement for a sync_file to complete in
> finite time, since the user process could be stopped or killed.
> 
> Any ideas?

Finite just means "not infinite". If you stop the process that's doing
part of the pipeline processing you block the pipeline, you get to keep
the pieces in that case. That's one of the issues with implicit sync
that explicit may solve: a single client taking way too much time to
render something can block the whole pipeline up until the display
flip. With explicit sync the compositor can just decide to use the last
client buffer if the latest buffer isn't ready by some deadline.

With regard to the process getting killed: whatever you sync primitive
is, you need to make sure to signal the fence (possibly with an error
condition set) when you are not going to make progress anymore. So
whatever your means to creating the sync_fd from your software renderer
is, it needs to signal any outstanding fences on the sync_fd when the
fd is closed.

Regards,
Lucas