Re: [PATCH v12 08/15] media: uapi: Define audio sample format fourcc type

[Shengjiu Wang]

On Wed, Feb 21, 2024 at 7:10 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>
> On 19/02/2024 13:56, Mauro Carvalho Chehab wrote:
> > Em Mon, 19 Feb 2024 12:05:02 +0800
> > Shengjiu Wang <shengjiu.wang@xxxxxxxxx> escreveu:
> >
> >> Hi Mauro
> >>
> >> On Sat, Feb 17, 2024 at 5:19 PM Mauro Carvalho Chehab
> >> <mchehab@xxxxxxxxxx> wrote:
> >>>
> >>> Em Thu, 18 Jan 2024 20:32:01 +0800
> >>> Shengjiu Wang <shengjiu.wang@xxxxxxx> escreveu:
> >>>
> >>>> The audio sample format definition is from alsa,
> >>>> the header file is include/uapi/sound/asound.h, but
> >>>> don't include this header file directly, because in
> >>>> user space, there is another copy in alsa-lib.
> >>>> There will be conflict in userspace for include
> >>>> videodev2.h & asound.h and asoundlib.h
> >>>>
> >>>> Here still use the fourcc format.
> >>>>
> >>>> Signed-off-by: Shengjiu Wang <shengjiu.wang@xxxxxxx>
> >>>> ---
> >>>>  .../userspace-api/media/v4l/pixfmt-audio.rst  | 87 +++++++++++++++++++
> >>>>  .../userspace-api/media/v4l/pixfmt.rst        |  1 +
> >>>>  drivers/media/v4l2-core/v4l2-ioctl.c          | 13 +++
> >>>>  include/uapi/linux/videodev2.h                | 23 +++++
> >>>>  4 files changed, 124 insertions(+)
> >>>>  create mode 100644 Documentation/userspace-api/media/v4l/pixfmt-audio.rst
> >>>>
> >>>> diff --git a/Documentation/userspace-api/media/v4l/pixfmt-audio.rst b/Documentation/userspace-api/media/v4l/pixfmt-audio.rst
> >>>> new file mode 100644
> >>>> index 000000000000..04b4a7fbd8f4
> >>>> --- /dev/null
> >>>> +++ b/Documentation/userspace-api/media/v4l/pixfmt-audio.rst
> >>>> @@ -0,0 +1,87 @@
> >>>> +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
> >>>> +
> >>>> +.. _pixfmt-audio:
> >>>> +
> >>>> +*************
> >>>> +Audio Formats
> >>>> +*************
> >>>> +
> >>>> +These formats are used for :ref:`audiomem2mem` interface only.
> >>>> +
> >>>> +.. tabularcolumns:: |p{5.8cm}|p{1.2cm}|p{10.3cm}|
> >>>> +
> >>>> +.. cssclass:: longtable
> >>>> +
> >>>> +.. flat-table:: Audio Format
> >>>> +    :header-rows:  1
> >>>> +    :stub-columns: 0
> >>>> +    :widths:       3 1 4
> >>>> +
> >>>> +    * - Identifier
> >>>> +      - Code
> >>>> +      - Details
> >>>> +    * .. _V4L2-AUDIO-FMT-S8:
> >>>> +
> >>>> +      - ``V4L2_AUDIO_FMT_S8``
> >>>> +      - 'S8'
> >>>> +      - Corresponds to SNDRV_PCM_FORMAT_S8 in ALSA
> >>>> +    * .. _V4L2-AUDIO-FMT-S16-LE:
> >>>
> >>> Hmm... why can't we just use SNDRV_*_FORMAT_*? Those are already part of
> >>> an uAPI header. No need to add any abstraction here and/or redefine
> >>> what is there already at include/uapi/sound/asound.h.
> >>>
> >> Actually I try to avoid including the include/uapi/sound/asound.h.
> >> Because in user space, there is another copy in alsa-lib (asoundlib.h).
> >> There will be conflict in userspace when including videodev2.h and
> >> asoundlib.h.
> >
> > Well, alsasoundlib.h seems to be using the same definitions:
> >       https://github.com/michaelwu/alsa-lib/blob/master/include/pcm.h
> >
> > So, I can't see what would be the actual issue, as both userspace library
> > and ALSA internal headers use the same magic numbers.
> >
> > You can still do things like:
> >
> >       #ifdef __KERNEL__
> >       #  include <sound/asound.h>
> >       #else
> >       #  include <asoundlib.h>
> >       #endif
> >
> > To avoid such kind of conflicts, if you need to have it included on
> > some header file. Yet, I can't see why you would need that.
> >
> > IMO, at uAPI headers, you just need to declare the uAPI audiofmt field
> > to be either __u32 or __u64, pointing it to where this value comes from
> > (on both userspace and Kernelspace. E. g.:
> >
> > /**
> >  * struct v4l2_audio_format - audio data format definition
> >  * @audioformat:
> >  *    an integer number matching the fields inside
> >  *    enum snd_pcm_format_t (e. g. `SNDRV_PCM_FORMAT_*`), as defined
> >  *    in include/uapi/sound/asound.h and
> >  *      https://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html#gaa14b7f26877a812acbb39811364177f8.
> >  * @channels:         channel numbers
> >  * @buffersize:               maximum size in bytes required for data
> >  */
> > struct v4l2_audio_format {
> >       __u32                           audioformat;
> >       __u32                           channels;
> >       __u32                           buffersize;
> > } __attribute__ ((packed));
> >
> > Then, at documentation you just need to point to where the
> > possible values for SNDRV_PCM_FORMAT_ are defined. No need to
> > document them one by one.
> >
> > With such definition, you'll only need to include sound/asound.h
> > within the kAPI scope.
> >
> >>
> >> And in the V4l framework, the fourcc type is commonly used in other
> >> cases (video, radio, touch, meta....), to avoid changing common code
> >> a lot, so I think using fourcc definition for audio may be simpler.
> >
> > Those are real video streams (or a video-related streams, in the case
> > of metadata) where fourcc is widely used. There, it makes sense.
> > However, ALSA format definitions are already being used for a long time.
> > There's no sense on trying to reinvent it - or having an abstract layer
> > to convert from/to fourcc <==> enum snd_pcm_format_t. Just use what is
> > there already.
>
> The problem is that within V4L2 we use fourcc consistently to describe a
> format, including in VIDIOC_ENUM_FMT. And the expectation is that the fourcc
> can be printed to a human readable string (there is even a printk format for
> that these days).
>
> But the pcm values are all small integers (and can even be 0!), and
> printing the fourcc will give garbage. It doesn't work well at all
> with the V4L2 API. But by having a straightforward conversion between the
> pcm identifier and a fourcc it was really easy to deal with this.
>
> There might even be applications today that call VIDIOC_ENUM_FMT to see
> what is supported and fail if it is not a proper fourcc is returned.
>
> It will certainly report nonsense in v4l_print_fmtdesc() (v4l2-ioctl.c).
>
> One of the early versions of this patch series did precisely what you request,
> but it just doesn't work well within the V4L2 uAPI.
>

Thanks all.

So can I reach the conclusion below?
1. Keep using the fourcc definition for v4l2_audio_format.audioformat.
2. Keep the name V4L2_BUF_TYPE_AUDIO_CAPTURE and
    V4L2_BUF_TYPE_AUDIO_OUTPUT.
3. Fix Documentation/userspace-api/media/v4l/dev-audio-mem2mem.rst
    to change 'pixelformat' to 'audioformat'.
4. Update Documentation/userspace-api/media/v4l/ext-ctrls-audio-m2m.rst
    to add more description for clock drift.

Best Regards
Shengjiu Wang

> Regards,
>
>         Hans
>
> >
> > Thanks,
> > Mauro
>