Re: [PATCH v1] audio/virtio-snd: fix latency calc

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Nov 15, 2025 at 12:08:07PM +0200, Manos Pitsidianakis wrote:
> On Fri, Nov 14, 2025 at 11:35 AM Yanfeng Liu <yfliu2008@qq.com> wrote:
> >
> > Media players needs meaningful latency_bytes update but it is
> > zero now most of the time. This adds stream-wise latency_bytes
> > calculation so that to improve the situation.
> >
> > Signed-off-by: Yanfeng Liu <p-liuyanfeng9@xiaomi.com>
> > ---

Manos what do you want to do with this patch?

> 
> Thanks for this patch. The virtio spec doesn't adequately explain what
> latency_bytes means, could you explain what it does and why it's
> needed? And also what behavior in media players this patch fixes?
> 
> Thanks again!
> 
> >  hw/audio/virtio-snd.c         | 12 +++++++++++-
> >  include/hw/audio/virtio-snd.h |  1 +
> >  2 files changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/audio/virtio-snd.c b/hw/audio/virtio-snd.c
> > index 9101560f38..ed0422b45a 100644
> > --- a/hw/audio/virtio-snd.c
> > +++ b/hw/audio/virtio-snd.c
> > @@ -431,6 +431,7 @@ static uint32_t virtio_snd_pcm_prepare(VirtIOSound *s, uint32_t stream_id)
> >          stream->id = stream_id;
> >          stream->pcm = s->pcm;
> >          stream->s = s;
> > +        stream->latency_bytes = 0;
> >          qemu_mutex_init(&stream->queue_mutex);
> >          QSIMPLEQ_INIT(&stream->queue);
> >
> > @@ -899,6 +900,7 @@ static void virtio_snd_handle_tx_xfer(VirtIODevice *vdev, VirtQueue *vq)
> >              buffer->vq = vq;
> >              buffer->size = size;
> >              buffer->offset = 0;
> > +            stream->latency_bytes += size;
> >
> >              QSIMPLEQ_INSERT_TAIL(&stream->queue, buffer, entry);
> >          }
> > @@ -1112,12 +1114,19 @@ error_cleanup:
> >      virtio_snd_unrealize(dev);
> >  }
> >
> > +static inline void update_latency(VirtIOSoundPCMStream *s, size_t used)
> > +{
> > +    s->latency_bytes = s->latency_bytes > used ?
> > +                       s->latency_bytes - used : 0;
> > +}
> > +
> >  static inline void return_tx_buffer(VirtIOSoundPCMStream *stream,
> >                                      VirtIOSoundPCMBuffer *buffer)
> >  {
> >      virtio_snd_pcm_status resp = { 0 };
> >      resp.status = cpu_to_le32(VIRTIO_SND_S_OK);
> > -    resp.latency_bytes = cpu_to_le32((uint32_t)buffer->size);
> > +    update_latency(stream, buffer->size);
> > +    resp.latency_bytes = cpu_to_le32(stream->latency_bytes);
> >      iov_from_buf(buffer->elem->in_sg,
> >                   buffer->elem->in_num,
> >                   0,
> > @@ -1178,6 +1187,7 @@ static void virtio_snd_pcm_out_cb(void *data, int available)
> >                  buffer->size -= size;
> >                  buffer->offset += size;
> >                  available -= size;
> > +                update_latency(stream, size);
> >                  if (buffer->size < 1) {
> >                      return_tx_buffer(stream, buffer);
> >                      break;
> > diff --git a/include/hw/audio/virtio-snd.h b/include/hw/audio/virtio-snd.h
> > index c176066584..9560bac8b1 100644
> > --- a/include/hw/audio/virtio-snd.h
> > +++ b/include/hw/audio/virtio-snd.h
> > @@ -150,6 +150,7 @@ struct VirtIOSoundPCMStream {
> >      } voice;
> >      QemuMutex queue_mutex;
> >      bool active;
> > +    uint32_t latency_bytes;
> >      QSIMPLEQ_HEAD(, VirtIOSoundPCMBuffer) queue;
> >  };
> >
> > --
> > 2.34.1
> >