Re: [PATCH] ALSA: virtio: reset I/O message fill level on prepare

[Takashi Iwai <tiwai@suse.de>]

On Sun, 11 Oct 2026 14:54:26 +0200,
Ting-Han Hou wrote:
> 
> Since commit fe981e67568c ("ALSA: virtio: use ack callback"), an I/O
> message is only queued to the device once the application has filled
> the whole period: virtsnd_pcm_msg_send() accumulates the transferred
> bytes in msg->length and queues the message when it reaches
> period_bytes.  msg->length is reset to 0 only when the device completes
> the message.
> 
> If the substream is stopped (e.g. snd_pcm_drop()) while a period is
> only partially filled, the corresponding message is never queued and
> keeps its partial length.  virtsnd_pcm_prepare() resets msg_count,
> hw_ptr and the indirect PCM state, but not msg->length.  After the
> next start, the stale length is added to the new data, the sum never
> equals period_bytes, and the message is never sent again.  Playback
> then stalls once hw_ptr reaches that period: writes fail with -EIO
> ("DMA or IRQ trouble") and drain fails with -EIO.  Capture is affected the
> same way after a partial read.
> 
> Clear the fill level of all messages in prepare.  Outside of the resume
> path, virtsnd_pcm_prepare() already refuses to run while I/O messages
> are pending, so no message is owned by the device at this point and the
> reset does not race with message completion.
> 
> Reproduced in QEMU (virtio-sound-pci, wav audiodev): prepare, write 1.5
> periods, drop, prepare, write 16 full periods, drain.  Without this
> patch WRITEI returns -EIO after 14 periods and DRAIN fails; with it all
> 16 periods are played and DRAIN succeeds.  Period-aligned prefill works
> in both cases.  The same sequence on capture (read 1.5 periods, drop,
> prepare, read 16 periods) fails with -EIO after 10 periods without this
> patch and reads all 16 with it.
> 
> Found by code inspection, confirmed with the reproducer above.
> 
> Fixes: fe981e67568c ("ALSA: virtio: use ack callback")
> Assisted-by: LLM
> Signed-off-by: Ting-Han Hou <ue081723@gmail.com>
> ---

Applied to for-next branch now.  Thanks.


Takashi