Re: Support for 2D engines/blitters in V4L2 and DRM

[Tomasz Figa]

On Fri, Apr 19, 2019 at 9:30 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote:
>
> Le jeudi 18 avril 2019 Ã 10:18 +0200, Daniel Vetter a Ãcrit :
> > > It would be cool if both could be used concurrently and not just return
> > > -EBUSY when the device is used with the other subsystem.
> >
> > We live in this world already :-) I think there's even patches (or merged
> > already) to add fences to v4l, for Android.
>
> This work is currently suspended. It will require some feature on DRM
> display to really make this useful, but there is also a lot of
> challanges in V4L2. In GFX space, most of the use case are about
> rendering as soon as possible. Though, in multimedia we have two
> problems, we need to synchronize the frame rendering with the audio,
> and output buffers may comes out of order due to how video CODECs are
> made.
>
> In the first, we'd need a mechanism where we can schedule a render at a
> specific time or vblank. We can of course already implement this in
> software, but with fences, the scheduling would need to be done in the
> driver. Then if the fence is signalled earlier, the driver should hold
> on until the delay is met. If the fence got signalled late, we also
> need to think of a workflow. As we can't schedule more then one render
> in DRM at one time, I don't really see yet how to make that work.
>
> For the second, it's complicated on V4L2 side. Currently we signal
> buffers when they are ready in the display order. With fences, we
> receive early pairs buffer and fence (in decoding order). There exist
> cases where reordering is done by the driver (stateful CODEC). We
> cannot schedule these immediately we would need a new mechanism to know
> which one come next. If we just reuse current mechnism, it would void
> the fence usage since the fence will always be signalled by the time it
> reaches DRM or other v4l2 component.
>
> There also other issues, for video capture pipeline, if you are not
> rendering ASAP, you need the HW timestamp in order to schedule. Again,
> we'd get the fence early, but the actual timestamp will be signalled at
> the very last minutes, so we also risk of turning the fence into pure
> overhead. Note that as we speak, I have colleagues who are
> experimenting with frame timestamp prediction that slaves to the
> effective timestamp (catching up over time). But we still have issues
> when the capture driver skipped a frame (missed a capture window).

Note that a fence has a timestamp internally and it can be queried for
it from the user space if exposed as a sync file:
https://elixir.bootlin.com/linux/v5.1-rc5/source/drivers/dma-buf/sync_file.c#L386

Fences in V4L2 would be also useful for stateless decoders and any
mem-to-mem processors that operate in order, like the blitters
mentioned here or actually camera ISPs, which can be often chained
into relatively sophisticated pipelines.

Best regards,
Tomasz