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

On 21/02/2024 11:11, Shengjiu Wang wrote:

On Wed, Feb 21, 2024 at 12:30 PM Tomasz Figa [off-list ref] wrote:

quoted

On Sat, Feb 17, 2024 at 6:42 PM Mauro Carvalho Chehab
[off-list ref] wrote:

quoted

Em Thu, 18 Jan 2024 20:32:00 +0800
Shengjiu Wang [off-list ref] escreveu:

quoted

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 <redacted>
---
 .../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

quoted

       - 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

quoted

quoted

quoted

+      - 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

quoted

quoted

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