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

[Daniel Vetter]

On Tue, Mar 17, 2020 at 12:18:47PM -0500, Jason Ekstrand wrote:
> 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?

vgem does that already (fences with timeout). The corruption issue is also
not new, if your shaders take forever real gpus will nick your rendering
with a quick reset. Iirc someone (from cros google team maybe) was even
looking into making llvmpipe run on top of vgem as a real dri/drm mesa
driver.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch