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ALSA: virtio: reset I/O message fill level on prepare

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Since commit fe981e6756 ("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: fe981e6756 ("ALSA: virtio: use ack callback")
Assisted-by: LLM
Signed-off-by: Ting-Han Hou <ue081723@gmail.com>
Link: https://patch.msgid.link/20261011125426.35745-1-ue081723@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Ting-Han Hou 2026-10-11 20:54:26 +08:00 committed by Takashi Iwai
parent f9e882ac82
commit f0e9db0f84
3 changed files with 22 additions and 0 deletions
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2
sound/virtio/virtio_pcm.h
View File

@ -118,6 +118,8 @@ int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss);
void virtsnd_pcm_msg_reset(struct virtio_pcm_substream *vss);
int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss, unsigned long offset,
unsigned long bytes);

19
sound/virtio/virtio_pcm_msg.c
View File

@ -182,6 +182,25 @@ void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss)
vss->nmsgs = 0;
}
/**
* virtsnd_pcm_msg_reset() - Reset the fill level of all I/O messages.
* @vss: VirtIO PCM substream.
*
* Messages that were only partially filled by the application when the
* substream was stopped are never sent to the device, so their length must be
* cleared before the substream is started again.
*
* Context: Any context. Takes and releases the VirtIO substream spinlock.
*/
void virtsnd_pcm_msg_reset(struct virtio_pcm_substream *vss)
{
unsigned int i;
guard(spinlock_irqsave)(&vss->lock);
for (i = 0; vss->msgs && i < vss->nmsgs; ++i)
vss->msgs[i]->length = 0;
}
/**
* virtsnd_pcm_msg_send() - Send asynchronous I/O messages.
* @vss: VirtIO PCM substream.

1
sound/virtio/virtio_pcm_ops.c
View File

@ -299,6 +299,7 @@ static int virtsnd_pcm_prepare(struct snd_pcm_substream *substream)
vss->xfer_xrun = false;
vss->suspended = false;
vss->msg_count = 0;
virtsnd_pcm_msg_reset(vss);
memset(&vss->pcm_indirect, 0, sizeof(vss->pcm_indirect));
vss->pcm_indirect.sw_buffer_size =