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

[Ting-Han Hou <ue081723@gmail.com>]

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>
---
Testing notes:
- Tested only with QEMU's virtio-sound-pci (wav and none audiodevs) on
  x86_64 with KASAN enabled, using a raw-ioctl ALSA test program;
  not tested with other device implementations (e.g. crosvm,
  vhost-user-sound) or with alsa-lib based players.
- Suspend/resume during playback (pm_test=devices) was also tried in
  QEMU and works with and without this patch.
- Lockdep was not enabled.

 sound/virtio/virtio_pcm.h     |  2 ++
 sound/virtio/virtio_pcm_msg.c | 19 +++++++++++++++++++
 sound/virtio/virtio_pcm_ops.c |  1 +
 3 files changed, 22 insertions(+)

diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
index 368dafa5df3f..8837dfc12828 100644
--- a/sound/virtio/virtio_pcm.h
+++ b/sound/virtio/virtio_pcm.h
@@ -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);
 
diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c
index a5c4e7027717..72103f606716 100644
--- a/sound/virtio/virtio_pcm_msg.c
+++ b/sound/virtio/virtio_pcm_msg.c
@@ -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.
diff --git a/sound/virtio/virtio_pcm_ops.c b/sound/virtio/virtio_pcm_ops.c
index 1105e7ff3523..1091100a4c98 100644
--- a/sound/virtio/virtio_pcm_ops.c
+++ b/sound/virtio/virtio_pcm_ops.c
@@ -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 =
-- 
2.53.0