[FFmpeg-devel] FFMPEG for V4L2 M2M devices ?

[Andrii]

>
> A quick Google implies that NVidia already has a stateful V4L2 M2M
> driver in their vendor kernel. Other than the strange choice of device
> node name (/dev/nvhost-nvdec), the details at [3] make it look like a
> normal V4L2 M2M decoder that has a good chance of working against
> h264_v4l2m2m.


Not only does it have a strange node name, it also uses two nodes. One for
decoding, another for converting. Capture plane of the decoder stores
frames in V4L2_PIX_FMT_NV12M format.
Converter able to convert it to a different format[1].

Could you point me at documentation of Pi V4L2 spec?

[1] https://docs.nvidia.com/jetson/l4t-multimedia/group__V4L2Conv.html

Andrii

On Mon, Jul 12, 2021 at 6:02 AM Dave Stevenson <
dave.stevenson at raspberrypi.com> wrote:

> On Sat, 10 Jul 2021 at 00:56, Brad Hards <bradh at frogmouth.net> wrote:
> >
> > On Saturday, 10 July 2021 8:53:27 AM AEST Andrii wrote:
> > > I am working on porting a Kodi player to an NVidia Jetson Nano device.
> I've
> > > been developing a decoder for quite some time now, and realized that
> the
> > > best approach would be to have it inside of ffmpeg, instead of
> embedding
> > > the decoder into Kodi as it heavily relies on FFMPEG. Just wondering if
> > > there is any effort in making FFMPEG suppring M2M V4L devices ?
> >
> >
> https://git.ffmpeg.org/gitweb/ffmpeg.git/blob_plain/HEAD:/libavcodec/v4l2_m2m.c[1]
> >
> > I guess that would be the basis for further work as required to meet
> your needs.
>
> Do note that there are 2 V4L2 M2M decoder APIs - the stateful API[1] ,
> and the stateless API [2]. They differ in the amount of bitstream
> parsing and buffer management that the driver implements vs expecting
> the client to do.
>
> The *_v4l2m2m drivers within FFMPEG support the stateful API (ie the
> kernel driver has bitstream parsing). For Raspberry Pi we use that to
> support the (older) H264 implementation, and FFMPEG master does that
> very well.
>
> The Pi HEVC decoder uses the V4L2 stateless API. Stateless HEVC
> support hasn't been merged to the mainline kernel as yet, so there are
> downstream patches to support that.
>
> A quick Google implies that NVidia already has a stateful V4L2 M2M
> driver in their vendor kernel. Other than the strange choice of device
> node name (/dev/nvhost-nvdec), the details at [3] make it look like a
> normal V4L2 M2M decoder that has a good chance of working against
> h264_v4l2m2m.
>
> [1]
> https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/dev-decoder.html
> [2]
> https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/dev-stateless-decoder.html
> [3] https://docs.nvidia.com/jetson/l4t-multimedia/group__V4L2Dec.html
>
>   Dave
>
> > Brad
> >
> > --------
> > [1]
> https://git.ffmpeg.org/gitweb/ffmpeg.git/blob_plain/HEAD:/libavcodec/v4l2_m2m.c
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