[PATCH v3 0/9] dma-fence: Deadline awareness

[Rob Clark]

On Thu, Sep 9, 2021 at 9:42 AM Simon Ser <contact at emersion.fr> wrote:
>
> On Thursday, September 9th, 2021 at 18:31, Rob Clark <robdclark at gmail.com> wrote:
>
> > Yes, I think it would.. and "dma-buf/sync_file: Add SET_DEADLINE
> > ioctl" adds such an ioctl.. just for the benefit of igt tests at this
> > point, but the thought was it would be also used by compositors that
> > are doing such frame scheduling. Ofc danvet is a bit grumpy that
> > there isn't a more real (than igt) userspace for the ioctl yet ;-)
>
> Ah, very nice, I somehow missed it.
>
> I guess one issue is that explicit sync isn't quite plumbed through
> compositors yet, so without Jason's DMA-BUF to sync_file IOCTL it'd be
> a bit difficult to use.
>
> Can anybody set the deadline? I wonder if clients should be allowed to.

In its current form, anyone who has the fd can.. I'm not sure how (or
even if) we could limit it beyond that.  I suppose hypothetically you
could use this for completely non-compositor related things, like
compute jobs where you want the result by some deadline.  (OTOH, it
could be the driver using this internally when the app is stalling on
a result)

> What happens if the deadline is exceeded? I'd assume nothing in
> particular, the deadline being just a hint?

Nothing in particular, it is just a hint.  The main intention is to
provide a feedback hint to the drivers in scenarios like vblank
deadlines, where being 1ms late is just as bad as being
$frame_duration-1ms late.  From my experiments and profiling it is
useful in a couple scenarios:

1) input latency, ie. go from idle to fullscreen animation, where GPU
has been idle for a while and not busy enough *yet* for devfreq to
kick in and start ramping up the freq before we miss the first vblank
2) double buffering, for ex. if you are 1ms late you end up stalling
15ms before the gpu can start rendering the next frame.. in the
absence of feedback, devfreq would ramp down in this scenario instead
of up

BR,
-R