Re: [PATCH v15 00/16] Add audio support in v4l2 framework

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Hey Shengjiu,

first of all thanks for all of this work and I am very sorry for only
emerging this late into the series, I sadly didn't notice it earlier.

I would like to voice a few concerns about the general idea of adding
Audio support to the Media subsystem.

1. The biggest objection is, that the Linux Kernel has a subsystem
specifically targeted for audio devices, adding support for these
devices in another subsystem are counterproductive as they work around
the shortcomings of the audio subsystem while forcing support for a
device into a subsystem that was never designed for such devices.
Instead, the audio subsystem has to be adjusted to be able to support
all of the required workflows, otherwise, the next audio driver with
similar requirements will have to move to the media subsystem as well,
the audio subsystem would then never experience the required change and
soon we would have two audio subsystems.

2. Closely connected to the previous objection, the media subsystem with
its current staff of maintainers is overworked and barely capable of
handling the workload, which includes an abundance of different devices
from DVB, codecs, cameras, PCI devices, radio tuners, HDMI CEC, IR
receivers, etc. Adding more device types to this matrix will make the
situation worse and should only be done with a plan for how first to
improve the current maintainer situation.

3. By using the same framework and APIs as the video codecs, the audio
codecs are going to cause extra work for the video codec developers and
maintainers simply by occupying the same space that was orginally
designed for the purpose of video only. Even if you try to not cause any
extra stress the simple presence of the audio code in the codebase is
going to cause restrictions.

The main issue here is that the audio subsystem doesn't provide a
mem2mem framework and I would say you are in luck because the media
subsystem has gathered a lot of shortcomings with its current
implementation of the mem2mem framework over time, which is why a new
implementation will be necessary anyway.

So instead of hammering a driver into the wrong destination, I would
suggest bundling our forces and implementing a general memory-to-memory
framework that both the media and the audio subsystem can use, that
addresses the current shortcomings of the implementation and allows you
to upload the driver where it is supposed to be.
This is going to cause restrictions as well, like mentioned in the
concern number 3, but with the difference that we can make a general
plan for such a framework that accomodates lots of use cases and each
subsystem can add their routines on top of the general framework.

Another possible alternative is to try and make the DRM scheduler more
generally available, this scheduler is the most mature and in fact is
very similar to what you and what the media devices need.
Which again just shows how common your usecase actually is and how a
general solution is the best long term solution.

Please notice that Daniel Almeida is currently working on something
related to this:

If the toplevel maintainers decide to add the patchset so be it, but I
wanted to voice my concerns and also highlight that this is likely going
to cause extra stress for the video codecs maintainers and the
maintainers in general. We cannot spend a lot of time on audio codecs,
as video codecs already fill up our available time sufficiently,
so the use of the framework needs to be conservative and cause as little
extra work as possible for the original use case of the framework.


On 19.03.2024 15:50, Shengjiu Wang wrote:
Audio signal processing also has the requirement for memory to
memory similar as Video.

This asrc memory to memory (memory ->asrc->memory) case is a non
real time use case.

User fills the input buffer to the asrc module, after conversion, then asrc
sends back the output buffer to user. So it is not a traditional ALSA playback
and capture case.

It is a specific use case,  there is no reference in current kernel.
v4l2 memory to memory is the closed implementation,  v4l2 current
support video, image, radio, tuner, touch devices, so it is not
complicated to add support for this specific audio case.

Because we had implemented the "memory -> asrc ->i2s device-> codec"
use case in ALSA.  Now the "memory->asrc->memory" needs
to reuse the code in asrc driver, so the first 3 patches is for refining
the code to make it can be shared by the "memory->asrc->memory"

The main change is in the v4l2 side, A /dev/vl4-audioX will be created,
user applications only use the ioctl of v4l2 framework.

Other change is to add memory to memory support for two kinds of i.MX ASRC

changes in v15:
- update MAINTAINERS for imx-asrc.c and vim2m-audio.c

changes in v14:
- document the reservation of 'AUXX' fourcc format.
- add v4l2_audfmt_to_fourcc() definition.

changes in v13
- change 'pixelformat' to 'audioformat' in dev-audio-mem2mem.rst
- add more description for clock drift in ext-ctrls-audio-m2m.rst
- Add "media: v4l2-ctrls: add support for fraction_bits" from Hans
 to avoid build issue for kernel test robot

changes in v12
- minor changes according to comments
- drop min_buffers_needed = 1 and V4L2_CTRL_FLAG_UPDATE flag
- drop bus_info

changes in v11
- add add-fixed-point-test-controls in vivid.
- add v4l2_ctrl_fp_compose() helper function for min and max

changes in v10
- remove FIXED_POINT type
- change code base on media: v4l2-ctrls: add support for fraction_bits
- fix issue reported by kernel test robot
- remove module_alias

changes in v9:
- add media controller support
- refine the vim2m-audio to support 8k<->16k conversion.

changes in v8:
- refine V4L2_CAP_AUDIO_M2M to be 0x00000008
- update doc for FIXED_POINT
- address comments for imx-asrc

changes in v7:
- add acked-by from Mark
- separate commit for fixed point, m2m audio class, audio rate controls
- use INTEGER_MENU for rate,  FIXED_POINT for rate offset
- remove used fmts
- address other comments for Hans

changes in v6:
- use m2m_prepare/m2m_unprepare/m2m_start/m2m_stop to replace
 m2m_start_part_one/m2m_stop_part_one, m2m_start_part_two/m2m_stop_part_two.
- fix warning by kernel test rebot
- remove some unused format V4L2_AUDIO_FMT_XX
- Get SNDRV_PCM_FORMAT from V4L2_AUDIO_FMT in driver.
- rename audm2m to viaudm2m.

changes in v5:
- remove V4L2_AUDIO_FMT_LPCM
- define audio pixel format like V4L2_AUDIO_FMT_S8...
- remove rate and format in struct v4l2_audio_format.
- updata document accordingly.

changes in v4:
- update document style
- separate V4L2_AUDIO_FMT_LPCM and V4L2_CAP_AUDIO_M2M in separate commit

changes in v3:
- Modify documents for adding audio m2m support
- Add audio virtual m2m driver
- Defined V4L2_AUDIO_FMT_LPCM format type for audio.
- Defined V4L2_CAP_AUDIO_M2M capability type for audio m2m case.
- with modification in v4l-utils, pass v4l2-compliance test.

changes in v2:
- decouple the implementation in v4l2 and ALSA
- implement the memory to memory driver as a platfrom driver
 and move it to driver/media
- move fsl_asrc_common.h to include/sound folder

Hans Verkuil (1):
 media: v4l2-ctrls: add support for fraction_bits

Shengjiu Wang (15):
 ASoC: fsl_asrc: define functions for memory to memory usage
 ASoC: fsl_easrc: define functions for memory to memory usage
 ASoC: fsl_asrc: move fsl_asrc_common.h to include/sound
 ASoC: fsl_asrc: register m2m platform device
 ASoC: fsl_easrc: register m2m platform device
 media: uapi: Add V4L2_CAP_AUDIO_M2M capability flag
 media: v4l2: Add audio capture and output support
 media: uapi: Define audio sample format fourcc type
 media: uapi: Add V4L2_CTRL_CLASS_M2M_AUDIO
 media: uapi: Add audio rate controls support
 media: uapi: Declare interface types for Audio
 media: uapi: Add an entity type for audio resampler
 media: vivid: add fixed point test controls
 media: imx-asrc: Add memory to memory driver
 media: vim2m-audio: add virtual driver for audio memory to memory

.../media/mediactl/media-types.rst            |   11 +
.../userspace-api/media/v4l/buffer.rst        |    6 +
.../userspace-api/media/v4l/common.rst        |    1 +
.../media/v4l/dev-audio-mem2mem.rst           |   71 +
.../userspace-api/media/v4l/devices.rst       |    1 +
.../media/v4l/ext-ctrls-audio-m2m.rst         |   59 +
.../userspace-api/media/v4l/pixfmt-audio.rst  |  100 ++
.../userspace-api/media/v4l/pixfmt.rst        |    1 +
.../media/v4l/vidioc-enum-fmt.rst             |    2 +
.../media/v4l/vidioc-g-ext-ctrls.rst          |    4 +
.../userspace-api/media/v4l/vidioc-g-fmt.rst  |    4 +
.../media/v4l/vidioc-querycap.rst             |    3 +
.../media/v4l/vidioc-queryctrl.rst            |   11 +-
.../media/videodev2.h.rst.exceptions          |    3 +
MAINTAINERS                                   |   17 +
.../media/common/videobuf2/videobuf2-v4l2.c   |    4 +
drivers/media/platform/nxp/Kconfig            |   13 +
drivers/media/platform/nxp/Makefile           |    1 +
drivers/media/platform/nxp/imx-asrc.c         | 1256 +++++++++++++++++
drivers/media/test-drivers/Kconfig            |   10 +
drivers/media/test-drivers/Makefile           |    1 +
drivers/media/test-drivers/vim2m-audio.c      |  793 +++++++++++
drivers/media/test-drivers/vivid/vivid-core.h |    2 +
.../media/test-drivers/vivid/vivid-ctrls.c    |   26 +
drivers/media/v4l2-core/v4l2-compat-ioctl32.c |    9 +
drivers/media/v4l2-core/v4l2-ctrls-api.c      |    1 +
drivers/media/v4l2-core/v4l2-ctrls-core.c     |   93 +-
drivers/media/v4l2-core/v4l2-ctrls-defs.c     |   10 +
drivers/media/v4l2-core/v4l2-dev.c            |   21 +
drivers/media/v4l2-core/v4l2-ioctl.c          |   66 +
drivers/media/v4l2-core/v4l2-mem2mem.c        |   13 +-
include/media/v4l2-ctrls.h                    |   13 +-
include/media/v4l2-dev.h                      |    2 +
include/media/v4l2-ioctl.h                    |   34 +
.../fsl => include/sound}/fsl_asrc_common.h   |   60 +
include/uapi/linux/media.h                    |    2 +
include/uapi/linux/v4l2-controls.h            |    9 +
include/uapi/linux/videodev2.h                |   50 +-
sound/soc/fsl/fsl_asrc.c                      |  144 ++
sound/soc/fsl/fsl_asrc.h                      |    4 +-
sound/soc/fsl/fsl_asrc_dma.c                  |    2 +-
sound/soc/fsl/fsl_easrc.c                     |  233 +++
sound/soc/fsl/fsl_easrc.h                     |    6 +-
43 files changed, 3145 insertions(+), 27 deletions(-)
create mode 100644 Documentation/userspace-api/media/v4l/dev-audio-mem2mem.rst
create mode 100644 Documentation/userspace-api/media/v4l/ext-ctrls-audio-m2m.rst
create mode 100644 Documentation/userspace-api/media/v4l/pixfmt-audio.rst
create mode 100644 drivers/media/platform/nxp/imx-asrc.c
create mode 100644 drivers/media/test-drivers/vim2m-audio.c
rename {sound/soc/fsl => include/sound}/fsl_asrc_common.h (60%)


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