Re: [RFC PATCH v2 0/7] Add audio support in v4l2 framework

Shengjiu Wang
2023-08-04T12:20:12Z

On Wed, Aug 2, 2023 at 8:28 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> On 02/08/2023 14:02, Shengjiu Wang wrote:
> > On Wed, Aug 2, 2023 at 7:22 PM Takashi Iwai <tiwai@suse.de> wrote:
> >>
> >> On Wed, 02 Aug 2023 09:32:37 +0200,
> >> Hans Verkuil wrote:
> >>>
> >>> Hi all,
> >>>
> >>> On 25/07/2023 08:12, Shengjiu Wang wrote:
> >>>> Audio signal processing has the requirement for memory to
> >>>> memory similar as Video.
> >>>>
> >>>> This patch is to add this support in v4l2 framework, defined
> >>>> new buffer type V4L2_BUF_TYPE_AUDIO_CAPTURE and
> >>>> V4L2_BUF_TYPE_AUDIO_OUTPUT, defined new format v4l2_audio_format
> >>>> for audio case usage.
> >>>>
> >>>> The created audio device is named "/dev/audioX".
> >>>>
> >>>> And add memory to memory support for two kinds of i.MX ASRC
> >>>> module
> >>>
> >>> Before I spend time on this: are the audio maintainers OK with doing
> >>> this in V4L2?
> >>>
> >>> I do want to have a clear statement on this as it is not something I
> >>> can decide.
> >>
> >> Well, I personally don't mind to have some audio capability in v4l2
> >> layer. But, the only uncertain thing for now is whether this is a
> >> must-have or not.
> >>
> >
> > Thanks, I am also not sure about this. I am also confused that why
> > there is no m2m implementation for audio in the kernel. Audio also
> > has similar decoder encoder post-processing as video.
> >
> >>
> >> IIRC, the implementation in the sound driver side was never done just
> >> because there was no similar implementation? If so, and if the
> >> extension to the v4l2 core layer is needed, shouldn't it be more
> >> considered for the possible other route?
> >>
> >
> > Actually I'd like someone could point me to the other route. I'd like to
> > try.
> >
> > The reason why I select to extend v4l2 for such audio usage is that v4l2
> > looks best for this audio m2m implementation. v4l2 is designed for m2m
> > usage. if we need implement another 'route', I don't think it can do better
> > that v4l2.
> >
> > I appreciate that someone can share his ideas or doable solutions.
> > And please don't ignore my request, ignore my patch.
>
> To give a bit more background: if it is decided to use the v4l API for this
> (and I have no objection to this from my side since API/framework-wise it is a
> good fit for this), then there are a number of things that need to be done to
> get this into the media subsystem:
>
> - documentation for the new uAPI
> - add support for this to v4l2-ctl
> - add v4l2-compliance tests for the new device
> - highly desirable: have a virtual driver (similar to vim2m) that supports this:
> it could be as simple as just copy input to output. This helps regression
> testing.
> - it might need media controller support as well. TBD.
>
> None of this is particularly complex, but taken all together it is a fair
> amount of work that also needs a lot of review time from our side.
>
> I want to add one more option to the mix: drivers/media/core/v4l2-mem2mem.c is
> the main m2m framework, but it relies heavily on the videobuf2 framework for
> the capture and output queues.
>
> The core vb2 implementation in drivers/media/common/videobuf2/videobuf2-core.c
> is independent of V4L2 and can be used by other subsystems (in our case, it is
> also used by the DVB API). It is a possibility to create an alsa version of
> v4l2-mem2mem.c that uses the core vb2 code with an ALSA uAPI on top.
>
> So in drivers/media/common/videobuf2/ you would have a videobuf2-alsa.c besides
> the already existing videobuf2-v4l2.c and -dvb.c.
>
> Perhaps parts of v4l2-mem2mem.c can be reused as well in that case, but I am
> not sure if it is worth the effort. I suspect copying it to an alsa-mem2mem.c
> and adapting it for alsa is easiest if you want to go that way.
>

Thanks.

Does this means that videobuf2-v4l2.c and v4l2-mem2mem.c are dedicate
for video device? if audio want to use v4l2 framework, need to create
videobuf2-alsa.c and alsa-mem2mem.c, but it may cause a lot of function
duplicate.

Best regards
Wang Shengjiu