Re: [RFC PATCH v3 6/9] media: v4l2: Add audio capture and output support

[Shengjiu Wang]

On Thu, Sep 14, 2023 at 6:17 PM Sakari Ailus <sakari.ailus@xxxxxx> wrote:
>
> Hi Shenjiu,
>
> Thanks for the update.
>
> On Thu, Sep 14, 2023 at 01:54:02PM +0800, 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.
> >
> > Defined V4L2_AUDIO_FMT_LPCM format type for audio.
>
> This would be nicer as a separate patch. Also see the related comments
> below.

OK,  will separate it.

>
> >
> > Defined V4L2_CAP_AUDIO_M2M capability type for audio memory
> > to memory case.
> >
> > The created audio device is named "/dev/v4l-audioX".
> >
> > Signed-off-by: Shengjiu Wang <shengjiu.wang@xxxxxxx>
> > ---
> >  .../userspace-api/media/v4l/audio-formats.rst | 15 +++++
> >  .../userspace-api/media/v4l/buffer.rst        |  6 ++
> >  .../userspace-api/media/v4l/dev-audio.rst     | 63 +++++++++++++++++++
> >  .../userspace-api/media/v4l/devices.rst       |  1 +
> >  .../media/v4l/pixfmt-aud-lpcm.rst             | 31 +++++++++
> >  .../userspace-api/media/v4l/pixfmt.rst        |  1 +
> >  .../media/v4l/vidioc-enum-fmt.rst             |  2 +
> >  .../userspace-api/media/v4l/vidioc-g-fmt.rst  |  4 ++
> >  .../media/v4l/vidioc-querycap.rst             |  3 +
> >  .../media/videodev2.h.rst.exceptions          |  2 +
> >  .../media/common/videobuf2/videobuf2-v4l2.c   |  4 ++
> >  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                | 25 ++++++++
> >  16 files changed, 263 insertions(+)
> >  create mode 100644 Documentation/userspace-api/media/v4l/audio-formats.rst
> >  create mode 100644 Documentation/userspace-api/media/v4l/dev-audio.rst
> >  create mode 100644 Documentation/userspace-api/media/v4l/pixfmt-aud-lpcm.rst
> >
> > diff --git a/Documentation/userspace-api/media/v4l/audio-formats.rst b/Documentation/userspace-api/media/v4l/audio-formats.rst
> > new file mode 100644
> > index 000000000000..bc52712d20d3
> > --- /dev/null
> > +++ b/Documentation/userspace-api/media/v4l/audio-formats.rst
> > @@ -0,0 +1,15 @@
> > +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
> > +
> > +.. _audio-formats:
> > +
> > +*************
> > +Audio Formats
> > +*************
> > +
> > +These formats are used for :ref:`audio` interface only.
> > +
> > +
> > +.. toctree::
> > +    :maxdepth: 1
> > +
> > +    pixfmt-aud-lpcm
> > diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> > index 04dec3e570ed..80cf2cb20dfe 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
> > +      - Buffer for audio output, see :ref:`audio`.
> >
> >
> >  .. _buffer-flags:
> > diff --git a/Documentation/userspace-api/media/v4l/dev-audio.rst b/Documentation/userspace-api/media/v4l/dev-audio.rst
> > new file mode 100644
> > index 000000000000..f9bcf0c7b056
> > --- /dev/null
> > +++ b/Documentation/userspace-api/media/v4l/dev-audio.rst
> > @@ -0,0 +1,63 @@
> > +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
> > +
> > +.. _audiodev:
> > +
> > +******************
> > +audio Interface
>
> Capital "A"?

OK,  will modify it.

>
> > +******************
>
> Too many asterisks (same a few lines above, too).

ok, will update it.

>
> > +
> > +The audio interface is implemented on audio device nodes. The audio device
> > +which uses application software for modulation or demodulation. This
> > +interface is intended for controlling and data streaming of such devices
> > +
> > +Audio devices are accessed through character device special files named
> > +``/dev/v4l-audio``
> > +
> > +Querying Capabilities
> > +=====================
> > +
> > +Device nodes supporting the audio capture and output 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.
> > +
> > +At least one of the read/write or streaming I/O methods must be supported.
> > +
> > +
> > +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
> > +      - ``rate``
> > +      - The sample rate, set by the application. The range is [5512, 768000].
> > +    * - __u32
> > +      - ``format``
> > +      - The sample format, set by the application. format is defined as
> > +        SNDRV_PCM_FORMAT_S8, SNDRV_PCM_FORMAT_U8, ...,
> > +    * - __u32
> > +      - ``channels``
> > +      - The channel number, set by the application. channel number range is
> > +        [1, 32].
> > +    * - __u32
> > +      - ``buffersize``
> > +      - Maximum buffer size in bytes required for data. The value is set by the
> > +        driver.
> > diff --git a/Documentation/userspace-api/media/v4l/devices.rst b/Documentation/userspace-api/media/v4l/devices.rst
> > index 8bfbad65a9d4..8261f3468489 100644
> > --- a/Documentation/userspace-api/media/v4l/devices.rst
> > +++ b/Documentation/userspace-api/media/v4l/devices.rst
> > @@ -24,3 +24,4 @@ Interfaces
> >      dev-event
> >      dev-subdev
> >      dev-meta
> > +    dev-audio
> > diff --git a/Documentation/userspace-api/media/v4l/pixfmt-aud-lpcm.rst b/Documentation/userspace-api/media/v4l/pixfmt-aud-lpcm.rst
> > new file mode 100644
> > index 000000000000..f9ebe2a05f69
> > --- /dev/null
> > +++ b/Documentation/userspace-api/media/v4l/pixfmt-aud-lpcm.rst
> > @@ -0,0 +1,31 @@
> > +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
> > +
> > +.. _v4l2-aud-fmt-lpcm:
> > +
> > +*************************
> > +V4L2_AUDIO_FMT_LPCM ('LPCM')
> > +*************************
> > +
> > +Linear Pulse-Code Modulation (LPCM)
> > +
> > +
> > +Description
> > +===========
> > +
> > +This describes audio format used by the audio memory to memory driver.
> > +
> > +It contains the following fields:
> > +
> > +.. flat-table::
> > +    :widths: 1 4
> > +    :header-rows:  1
> > +    :stub-columns: 0
> > +
> > +    * - Field
> > +      - Description
> > +    * - u32 samplerate;
> > +      - which is the number of times per second that samples are taken.
> > +    * - u32 sampleformat;
> > +      - which determines the number of possible digital values that can be used to represent each sample
>
> 80 characters (or less) per line, please.

Ok, will change it.

>
> Which values could this field have and what do they signify?

The values are SNDRV_PCM_FORMAT_S8, SNDRV_PCM_FORMAT_U8...
which are the PCM format, defined in ALSA.

>
> > +    * - u32 channels;
> > +      - channel number for each sample.
>
> I suppose the rest of the buffer would be samples? This should be
> documented. I think there are also different ways the data could be
> arrangeed and this needs to be documented, too.

All data in the buffer are the samples,  the 'samplerate', 'sampleformat'
'channels'  I list here is try to describe the samples.
I was confused how to write this document, so I list the characters.

Best regards
Wang Shengjiu