kmssink and ultra low latency

[Peter Allen]

Fantastic, thank you both. The pipeline is now working beautifully, and I'm
very happy with the decode/display latency. Now on to fixing latency in the
rest of the pipeline...

I'm going no sync for now, but will try playing with sync at some point to
see if it looks better.

Thank you
Pete

On Mon, 21 Nov 2022, 20:50 Nirbheek Chauhan, <nirbheek.chauhan at gmail.com>
wrote:

> On Tue, Nov 22, 2022 at 12:30 AM Nicolas Dufresne via gstreamer-devel
> <gstreamer-devel at lists.freedesktop.org> wrote:
> > Le lundi 21 novembre 2022 à 13:24 +0000, Peter Allen via gstreamer-devel
> a
> > écrit :
> > > I have an application that needs ultra-low latency decoding and
> display of a
> > > 720p/60 H264 rtp stream. I'm using Allwinner H3 (currently an Orange
> Pi PC,
> > > but that will change to a different H3 platform in due course), and
> displaying
> > > on a 1080p/60 monitor connected via hdmi.
> > >
> > > My current gstreamer pipeline is gst-launch-1.0 -v udpsrc port=5600 !
> > > "application/x-rtp, media=(string)video, clock-rate=(int)90000,
> encoding-
> > > name=(string)H264, payload=(int)96" ! rtph264depay ! h264parse !
> v4l2slh264dec
> > > ! kmssink plane-id=31 plane-properties=s,zpos=3
> > >
> > > if I change to fakesink, the decode is working beautifully (and
> according to
> > > gstreamer latency stats is giving a decode time  of around 5ms -
> better than I
> > > had hoped!) However I am having issues with kmssink. Initially I was
> only
> > > getting 30fps display, but applying the kmssink fix for atomic drm
> clients
> > > from
> https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/issues/654 and
> > > also the rockchip kernel fix I get smooth 60fps decoding, but only if
> I add a
> > > ! queue ! element between  v4l2slh264dec and kmssink which pushes the
> latency
> > > up unacceptably high.
> > >
> > > Playing with ! queue ! output buffers (max-size-buffers=1
> leaky=downstream)
> > > shows it needs max-size-buffers=3 buffers before it displays smoothly.
> (at
> > > max-size-buffers=1 I alternately 1 frame dropped then 2 frames
> dropped, at
> > > max-size-buffers=2 it's one frame dropped then no frames dropped)
> >
> > I can only presume that render time correctness does not matter to you.
> So my
> > advise would be to use kmssink sync=0.
>
> Another thing to try before turning off sync is changing
> processing-deadline to a lower value. Honestly the default 20ms is a
> bit too large.
>
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