Re: [PATCH v8 00/12] Add VIRTIO sound card

["Alex Bennée" <alex.bennee@linaro.org>]

Manos Pitsidianakis <manos.pitsidianakis@linaro.org> writes:

> Hello Volker :)
>
> On Mon, 04 Sep 2023 10:20, Volker Rümelin <vr_qemu@t-online.de> wrote:
>>All qemu_log_mask() format strings need a trailing \n.
>
> Thank you, will fix it!
>
>> I still hear a lot of playback dropouts. I had planned to look at
>> the playback code, but I didn't have the time until now.
>>
>> Compared to v6 audio recording has improved but there are bugs. When
>> I start QEMU with -audiodev
>> pipewire,out.frequency=48000,in.frequency=48000,id=audio0 there are
>> two either uninitialized or stale samples every 25ms in the recorded
>> audio stream.
>>
>> To reproduce the issue start audacity on the host and generate a 2s
>> square wave tone with 315Hz and an amplitude of 0.8. Use pavucontrol
>> to select the monitor of your host playback device as QEMU recording
>> device. In the guest start recording with audacity. Start playback
>> of the generated square wave on the host. Stop recording in the
>> guest and have a look at a 200ms sequence of the recorded square
>> wave and notice the wrong samples every 25ms.
>
> We've noticed this and decided to fix it in the future. I think the
> problem lies when PCM release is called from the guest. Quoting the
> spec:
>
>  The device MUST complete all pending I/O messages for the specified
>  stream ID.
>  The device MUST NOT complete the control request while there are
>  pending I/O messages for the specified stream ID.
>
> When RELEASE is received, buffers are simply dropped. This is pure
> conjecture but I think creating an in-device buffer could solve this.
> Unless the bug is found to be caused by something else, I settled on
> accepting it for this patch series because it is spec conformant.

Volker,

Can you run with:

  -d trace:virtio_snd\*

to confirm you are seeing the same behaviour. The experience I had with
ogg123 in an emulated guest was it would work fine but then the next run
I would get audio corruption. You can see this if you see lots of
START/STOP/RELEASE messages constantly restarting things. If you are
getting corruption without this pattern that is something else which we
should investigate before merging.

>
>> When I start QEMU with -audiodev
>> pipewire,out.mixing-engine=off,in.mixing-engine=off,id=audio0 audio
>> recording starts but the recorded stream immediately stalls.
>
> Can you elaborate? Do you mean you repeat the same process as before,
> but the stall happens immediately? I personally rarely get any drops I
> could notice, only one or two for many minutes of playback / capture.
> I also could not reproduce exactly the same behavior you had in the
> previous version. The bugs *were* there but it was not as severe.
> Maybe it's a hardware performance issue? Can someone else test this
> too? It'd be helpful.
>
> Thank you very much for your help,
> Manos


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro