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

[Lucas Stach]

Am Dienstag, den 17.03.2020, 10:59 -0700 schrieb Jacob Lifshay:
> On Tue, Mar 17, 2020 at 10:21 AM Lucas Stach <dev at lynxeye.de> wrote:
> > 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.
> 
> Seems reasonable.
> 
> > 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.
> 
> I think I found a userspace-accessible way to create sync_files and
> dma_fences that would fulfill the requirements:
> https://github.com/torvalds/linux/blob/master/drivers/dma-buf/sw_sync.c
> 
> I'm just not sure if that's a good interface to use, since it appears
> to be designed only for debugging. Will have to check for additional
> requirements of signalling an error when the process that created the
> fence is killed.

Something like that can certainly be lifted for general use if it makes
sense. But then with a software renderer I don't really see how fences
help you at all. With a software renderer you know exactly when the
frame is finished and you can just defer pushing it over to the next
pipeline element until that time. You won't gain any parallelism by
using fences as the CPU is busy doing the rendering and will not run
other stuff concurrently, right?

Regards,
Lucas