Re: [PATCH v12 07/15] media: v4l2: Add audio capture and output support

[Tomasz Figa]

On Sat, Feb 17, 2024 at 6:42 PM Mauro Carvalho Chehab
<mchehab@xxxxxxxxxx> wrote:
>
> Em Thu, 18 Jan 2024 20:32:00 +0800
> Shengjiu Wang <shengjiu.wang@xxxxxxx> escreveu:
>
> > 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/v4l-audioX".
> >
> > Signed-off-by: Shengjiu Wang <shengjiu.wang@xxxxxxx>
> > ---
> >  .../userspace-api/media/v4l/buffer.rst        |  6 ++
> >  .../media/v4l/dev-audio-mem2mem.rst           | 71 +++++++++++++++++++
> >  .../userspace-api/media/v4l/devices.rst       |  1 +
> >  .../media/v4l/vidioc-enum-fmt.rst             |  2 +
> >  .../userspace-api/media/v4l/vidioc-g-fmt.rst  |  4 ++
> >  .../media/videodev2.h.rst.exceptions          |  2 +
> >  .../media/common/videobuf2/videobuf2-v4l2.c   |  4 ++
> >  drivers/media/v4l2-core/v4l2-compat-ioctl32.c |  9 +++
> >  drivers/media/v4l2-core/v4l2-dev.c            | 17 +++++
> >  drivers/media/v4l2-core/v4l2-ioctl.c          | 53 ++++++++++++++
> >  include/media/v4l2-dev.h                      |  2 +
> >  include/media/v4l2-ioctl.h                    | 34 +++++++++
> >  include/uapi/linux/videodev2.h                | 17 +++++
> >  13 files changed, 222 insertions(+)
> >  create mode 100644 Documentation/userspace-api/media/v4l/dev-audio-mem2mem.rst
> >
> > diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> > index 52bbee81c080..a3754ca6f0d6 100644
> > --- a/Documentation/userspace-api/media/v4l/buffer.rst
> > +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> > @@ -438,6 +438,12 @@ enum v4l2_buf_type
> >      * - ``V4L2_BUF_TYPE_META_OUTPUT``
> >        - 14
>
> >        - Buffer for metadata output, see :ref:`metadata`.
> > +    * - ``V4L2_BUF_TYPE_AUDIO_CAPTURE``
> > +      - 15
> > +      - Buffer for audio capture, see :ref:`audio`.
> > +    * - ``V4L2_BUF_TYPE_AUDIO_OUTPUT``
> > +      - 16
>
> Hmm... alsa APi define input/output as:
>         enum {
>                 SNDRV_PCM_STREAM_PLAYBACK = 0,
>                 SNDRV_PCM_STREAM_CAPTURE,
>                 SNDRV_PCM_STREAM_LAST = SNDRV_PCM_STREAM_CAPTURE,
>         };
>
>
> I would use a namespace as close as possible to the
> ALSA API. Also, we're not talking about V4L2, but, instead
> audio. so, not sure if I like the prefix to start with
> V4L2_. Maybe ALSA_?
>
> So, a better namespace would be:
>
>         ${prefix}_BUF_TYPE_PCM_STREAM_PLAYBACK
> and
>         ${prefix}_BUF_TYPE_PCM_STREAM_CAPTURE
>

The API is still V4L2, and all the other non-video buf types also use
the V4L2_ prefix, so perhaps that's good here as well?

Whether AUDIO or PCM_STREAM makes more sense goes outside of my
expertise. Subjectively, a PCM stream sounds more specific than an
audio stream. Do those buf types also support non-PCM audio streams?

> > +      - Buffer for audio output, see :ref:`audio`.
> >
> >
> >  .. _buffer-flags:
> > diff --git a/Documentation/userspace-api/media/v4l/dev-audio-mem2mem.rst b/Documentation/userspace-api/media/v4l/dev-audio-mem2mem.rst
> > new file mode 100644
> > index 000000000000..68faecfe3a02
> > --- /dev/null
> > +++ b/Documentation/userspace-api/media/v4l/dev-audio-mem2mem.rst
> > @@ -0,0 +1,71 @@
> > +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
> > +
> > +.. _audiomem2mem:
> > +
> > +********************************
> > +Audio Memory-To-Memory Interface
> > +********************************
> > +
> > +An audio memory-to-memory device can compress, decompress, transform, or
> > +otherwise convert audio data from one format into another format, in memory.
> > +Such memory-to-memory devices set the ``V4L2_CAP_AUDIO_M2M`` capability.
> > +Examples of memory-to-memory devices are audio codecs, audio preprocessing,
> > +audio postprocessing.
> > +
> > +A memory-to-memory audio node supports both output (sending audio frames from
> > +memory to the hardware) and capture (receiving the processed audio frames
> > +from the hardware into memory) stream I/O. An application will have to
> > +setup the stream I/O for both sides and finally call
> > +:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` for both capture and output to
> > +start the hardware.
> > +
> > +Memory-to-memory devices function as a shared resource: you can
> > +open the audio node multiple times, each application setting up their
> > +own properties that are local to the file handle, and each can use
> > +it independently from the others. The driver will arbitrate access to
> > +the hardware and reprogram it whenever another file handler gets access.
> > +
> > +Audio memory-to-memory devices are accessed through character device
> > +special files named ``/dev/v4l-audio``
> > +
> > +Querying Capabilities
> > +=====================
> > +
> > +Device nodes supporting the audio memory-to-memory interface set the
> > +``V4L2_CAP_AUDIO_M2M`` flag in the ``device_caps`` field of the
> > +:c:type:`v4l2_capability` structure returned by the :c:func:`VIDIOC_QUERYCAP`
> > +ioctl.
> > +
> > +Data Format Negotiation
> > +=======================
> > +
> > +The audio device uses the :ref:`format` ioctls to select the capture format.
> > +The audio buffer content format is bound to that selected format. In addition
> > +to the basic :ref:`format` ioctls, the :c:func:`VIDIOC_ENUM_FMT` ioctl must be
> > +supported as well.
> > +
> > +To use the :ref:`format` ioctls applications set the ``type`` field of the
> > +:c:type:`v4l2_format` structure to ``V4L2_BUF_TYPE_AUDIO_CAPTURE`` or to
> > +``V4L2_BUF_TYPE_AUDIO_OUTPUT``. Both drivers and applications must set the
> > +remainder of the :c:type:`v4l2_format` structure to 0.
> > +
> > +.. c:type:: v4l2_audio_format
> > +
> > +.. tabularcolumns:: |p{1.4cm}|p{2.4cm}|p{13.5cm}|
> > +
> > +.. flat-table:: struct v4l2_audio_format
> > +    :header-rows:  0
> > +    :stub-columns: 0
> > +    :widths:       1 1 2
> > +
> > +    * - __u32
> > +      - ``pixelformat``
> > +      - The sample format, set by the application. see :ref:`pixfmt-audio`
>
> pixelformat doesn't make any sense for audio: there are no pixels on a
> PCM stream. Please use call it, instead: `snd_pcm_format`, making it match
> the values for snd_pcm_format_t.
>

+1

FWIW v4l2_meta_format uses the name "dataformat".

Actually, I just realized that the C code actually uses the name
"audioformat". Tbh., after reading the kerneldoc comment, my
subjective preference would be on "sample_format", since that's
exactly what it is.

> Yet, I would keep defining it as u32 (or u64?) instead of using a
> typedef int field there (snd_pcm_format_t), as the size of integer
> is different on 32 and 64 bit kernels.

+1

Best regards,
Tomasz