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Subject: Re: [virtio-dev] Request for a new device number for a virtio-audio device.

Il 29/04/19 15:47, Stefan Hajnoczi ha scritto:
On Mon, Apr 29, 2019 at 12:22:41AM +0200, Marco Martinelli - 13Byte srl wrote:
Hi everyone,

my name is Marco, from Italy. I'm a developer that recently got an interest
in improving the current situation of audio in qemu/kvm.

I've been working successfully on multiple patches for Scream
<https://github.com/duncanthrax/scream> ), a virtual sound card for
Microsoft Windows, that allow to transfer audio from a Windows VM to a Linux
host. The project initially supported only stereo sound over the network,
but with my patches now it support up to 7.1 surround and it can transfer
the audio samples using shared memory between the VM and the host (based on
IVSHMEM), for lower latency.

Now that was a good project, but I want to do more. I dream of an
virtio-audio device that can handle multichannel audio in and out of the VM,
to be used with more guest OSs than just Windows.

I know I have a long way to go to reach this goal, but I'm motivated and
willing to learn. I admit I don't have much experience with the qemu source
code or the virtio source code, but I learn fast.

My work on Scream was to familiarize with Windows driver development,
VM-to-host communication and low latency audio processing.
My idea is to start with small self-contained projects to familiarize with
the source codes of the various projects and gain the experience I lack,
then moving on the real virtio-audio project.
This way I can both obtain more knowledge on the subjects and help the open
source community that over the years have done so much for many.

As we speak I'm reading the VirtIO documentation, where I've found out that
I need to require a new device number for this project (Appendix B.3). So,
here I am.

I'm not quite sure if this email is enough to obtain a new number or if I
need to follow some process I'm not aware of. In the meantime I'm using the
device number 65535 as documented in the specifications.

Looking around I've seen there is a recent issue on GitHub to reserve a
virtio-audio device id, and a patch was submitter to assign id 25 to audio
Is this the same virtio-audio device type that you want to work on or is
this an independent effort from another author?  It seems important to
contact the author (if it's not you) to find out the status of their
device and see what work can be shared.

It is not my device, but I don't think there is any work behind this either.

The number 25 I was referring to comes from this message https://lists.oasis-open.org/archives/virtio/201903/msg00074.html

It seems to me that this number was reserved for a virtio-audio device, but not a specific one. Maybe I'm misinterpreting that message.

I'm not sure how this works, is that number already assigned and I should
use that or should I get a new one?

For last, I have a question to clear the things up for me. It is my
understanding that in this mailing list you discuss about the
specifications, not the actual code. What's the usual process when writing a
new virtio device?
Should I start with writing the code and then document how it works or is it
the opposite? Should I document it and have it approved and then implement
the specifications?

I know that this may sound like a stupid question, but please be patient
with me, it's my first time.
I suggest posting a draft specification for the new device type before
investing too much time in the implementation.

Then work on the code while the spec discussion is ongoing.  This way
you won't have to wait too long before implementation but you also won't
find that reviewers are asking for large changes after you completed
your work.
I feared that would be the answer.
As I stated I'm not familiar with most of the technologies involved. I don't know if I have the capabilities to write a draft of the specifications without first working on some code to clear things up.
I'll try my best.

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