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

[Hans Verkuil]

On 21/02/2024 11:11, Shengjiu Wang wrote:
> On Wed, Feb 21, 2024 at 12:30 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
>>
>> 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?
> 
> Currently I use it for PCM,  but I think it can also be used for non-PCM.
> So use the below name?
> V4L2_BUF_TYPE_AUDIO_CAPTURE
> V4L2_BUF_TYPE_AUDIO_PLAYBACK

I really prefer keeping the names as they are in this patch. CAPTURE/OUTPUT
is consistent with V4L2 nomenclature, and since this is a M2M device 'PLAYBACK'
isn't really a good name either. It's not an audio playback device, it's a
rate converter.

Regards,

	Hans

> 
>>
>>>> +      - 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.
>>
> Ok, I will change it to sampleformat.
> 
> Best Regards
> Shengjiu Wang
> 
>>> 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