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

[Hans Verkuil]

On 22/02/2024 04:50, Shengjiu Wang wrote:
> 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.

Yes, let's go with that.

Regards,

	Hans

> 
> Best Regards
> Shengjiu Wang
> 
>> Regards,
>>
>>         Hans
>>
>>>
>>> Thanks,
>>> Mauro
>>