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Date: Tue, 27 Feb 2024 09:34:20 +0100
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Subject: Re: [PATCH v13 09/16] media: uapi: Define audio sample format fourcc
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To: Shengjiu Wang <shengjiu.wang@gmail.com>,
 Nicolas Dufresne <nicolas@ndufresne.ca>
Cc: Shengjiu Wang <shengjiu.wang@nxp.com>, sakari.ailus@iki.fi,
 tfiga@chromium.org, m.szyprowski@samsung.com, mchehab@kernel.org,
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From: Hans Verkuil <hverkuil@xs4all.nl>
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On 27/02/2024 04:44, Shengjiu Wang wrote:
> On Mon, Feb 26, 2024 at 9:55 PM Nicolas Dufresne <nicolas@ndufresne.ca> wrote:
>>
>> Le lundi 26 février 2024 à 16:28 +0800, Shengjiu Wang a écrit :
>>> 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.
>>
>> I'd like to join Mauro's voice that duplicating the audio formats is a bad idea.
>> We have the same issues with video formats when you look at V4L2 vs DRM. You're
>> rationale is that videodev2.h will be ambiguous if it includes asound.h, but
>> looking at this change, there is no reason why you need to include asound.h in
>> videodev2.h at all. The format type can be abstracted out with a uint32 in the
>> API, and then it would be up to the users to include and use the appropriate
>> formats IDs.
>>
> 
> Resend for the plain text issue
> 
> Thanks.
> 
> There is another reason mentioned by Hans:
> 
> "
> 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.
> "

Right, and remember that the fourcc codes are really meant to be shown as
characters. v4l2-ctl and friends all use this, and we all talk about them
as such (NV12, YUYV, etc.). If I would just stick in the standard PCM IDs,
then they would all show up as '....'.

<snip>

>>> +/*
>>> + * Audio-data formats
>>> + * All these audio formats use a fourcc starting with 'AU'
>>> + * followed by the SNDRV_PCM_FORMAT_ value from asound.h.
>>> + */
>>> +#define V4L2_AUDIO_FMT_S8                    v4l2_fourcc('A', 'U', '0', '0')
>>> +#define V4L2_AUDIO_FMT_S16_LE                        v4l2_fourcc('A', 'U', '0', '2')
>>> +#define V4L2_AUDIO_FMT_U16_LE                        v4l2_fourcc('A', 'U', '0', '4')
>>> +#define V4L2_AUDIO_FMT_S24_LE                        v4l2_fourcc('A', 'U', '0', '6')
>>> +#define V4L2_AUDIO_FMT_U24_LE                        v4l2_fourcc('A', 'U', '0', '8')
>>> +#define V4L2_AUDIO_FMT_S32_LE                        v4l2_fourcc('A', 'U', '1', '0')
>>> +#define V4L2_AUDIO_FMT_U32_LE                        v4l2_fourcc('A', 'U', '1', '2')
>>> +#define V4L2_AUDIO_FMT_FLOAT_LE                      v4l2_fourcc('A', 'U', '1', '4')
>>> +#define V4L2_AUDIO_FMT_IEC958_SUBFRAME_LE    v4l2_fourcc('A', 'U', '1', '8')
>>> +#define V4L2_AUDIO_FMT_S24_3LE                       v4l2_fourcc('A', 'U', '3', '2')
>>> +#define V4L2_AUDIO_FMT_U24_3LE                       v4l2_fourcc('A', 'U', '3', '4')
>>> +#define V4L2_AUDIO_FMT_S20_3LE                       v4l2_fourcc('A', 'U', '3', '6')
>>> +#define V4L2_AUDIO_FMT_U20_3LE                       v4l2_fourcc('A', 'U', '3', '8')
>>> +
>>> +#define v4l2_fourcc_to_audfmt(fourcc)        \
>>> +     (__force snd_pcm_format_t)(((((fourcc) >> 16) & 0xff) - '0') * 10  \
>>> +                                + ((((fourcc) >> 24) & 0xff) - '0'))
>>> +

We did add a #define to go from a fourcc to an audio format.

Would it help to add a corresponding v4l2_audfmt_to_fourcc define as well?

#define v4l2_audfmt_to_fourcc(audfmt) \
	v4l2_fourcc('A', 'U', '0' + (audfmt) / 10, '0' + (audfmt) % 10)

I would have no problem with that.

The mapping between the audio format and the fourcc is very simple, it is just
converted to something that can be safely shown as ASCII characters.

Regards,

	Hans

>>>  /* priv field value to indicates that subsequent fields are valid. */
>>>  #define V4L2_PIX_FMT_PRIV_MAGIC              0xfeedcafe
>>>
>>