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

[Hans Verkuil]

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.

Regards,

	Hans

> 
> Thanks,
> Mauro