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

[Marco Martinelli - 13Byte srl <marco@13byte.com>]

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
> > <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
> > devices.
> 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.
> Stefan