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

[Jacob Lifshay]

On Tue, Mar 17, 2020 at 11:14 AM Lucas Stach <dev at lynxeye.de> wrote:
>
> Am Dienstag, den 17.03.2020, 10:59 -0700 schrieb Jacob Lifshay:
> > 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?

There definitely may be other hardware and/or processes that can
process some stuff concurrently with the main application, such as the
compositor and or video encoding processes (for video capture).
Additionally, from what I understand, sync_file is the standard way to
export and import explicit synchronization between processes and
between drivers on Linux, so it seems like a good idea to support it
from an interoperability standpoint even if it turns out that there
aren't any scheduling/timing benefits.

Jacob