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

[Jason Ekstrand]

On Tue, Mar 17, 2020 at 12:13 PM Jacob Lifshay <programmerjake at gmail.com> wrote:
>
> 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.

Yeah... That's going to be a problem.  The only way I could see that
working is if you created a sync_file that had a timeout associated
with it.  However, then you run into the issue where you may have
corruption if stuff doesn't complete on time.  Then again, you're not
really dealing with an external unit and so the latency cost of going
across the window system protocol probably isn't massively different
from the latency cost of triggering the sync_file.  Maybe the answer
there is to just do everything in-order and not worry about
synchronization?