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

[Hans Verkuil]

On 30/04/2024 10:21, Sebastian Fricke wrote:
> 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:
> https://lore.kernel.org/linux-media/3F80AC0D-DCAA-4EDE-BF58-BB1369C7EDCA@collabora.com/T/#u
> 
> 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.

I would really like to get the input of the audio maintainers on this.
Sebastian has a good point, especially with us being overworked :-)

Having a shared mem2mem framework would certainly be nice, on the other
hand, developing that will most likely take a substantial amount of time.

Perhaps it is possible to copy the current media v4l2-mem2mem.c and turn
it into an alsa-mem2mem.c? I really do not know enough about the alsa
subsystem to tell if that is possible.

While this driver is a rate converter, not an audio codec, the same
principles would apply to off-line audio codecs as well. And it is true
that we definitely do not want to support audio codecs in the media
subsystem.

Accepting this driver creates a precedent and would open the door for
audio codecs.

I may have been too hasty in saying yes to this, I did not consider
the wider implications for our workload and what it can lead to. I
sincerely apologize to Shengjiu Wang as it is no fun to end up in a
situation like this.

Regards,

	Hans