On Fri, Nov 14, 2025 at 11:35 AM Yanfeng Liu <[email protected]> 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 <[email protected]>
> ---
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
>
