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

Em Mon, 19 Feb 2024 12:05:02 +0800
Shengjiu Wang [off-list ref] escreveu:

Hi Mauro

On Sat, Feb 17, 2024 at 5:19 PM Mauro Carvalho Chehab
[off-list ref] wrote:

quoted

Em Thu, 18 Jan 2024 20:32:01 +0800
Shengjiu Wang [off-list ref] escreveu:
 

quoted

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 <redacted>
---
 .../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.

Thanks,
Mauro