[PATCH v24 16/16] ALSA: virtio: preserve playback progress across pause

[Joonho Lee <mynameisjune111@gmail.com>]

TX completions can arrive after PAUSE closes transfer admission.
Preserve that progress and schedule catch-up when the stream resumes or
a rejected pause restores RUNNING. Cache device latency in stream-owned
state so a late completion does not need to dereference the ALSA
runtime. Clear the cache on prepare and RESET.

Use the existing indirect hw_queue_size to keep outstanding TX below one
physical ring: buffer_bytes - period_bytes. Equal modulo cursors then
mean no unobserved completion, and a non-interrupt HWSYNC cannot consume
a hidden full-ring wrap before deferred period work. Continue using the
ordinary ALSA pointer update for silence, drain, stop-threshold and
wakeup policy.

This also reduces queued buffering by one period. With two periods, only
one period can be queued. The minimum geometry and pause/reopen cases
pass in QEMU, but worst-case scheduling tolerance has not been
qualified.

Assisted-by: LLM
Signed-off-by: Joonho Lee <mynameisjune111@gmail.com>

---
sound/virtio/virtio_pcm.h     |    2 ++
 sound/virtio/virtio_pcm_msg.c |    8 +++-----
 sound/virtio/virtio_pcm_ops.c |   25 ++++++++++++++++++++++---
 3 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
--- a/sound/virtio/virtio_pcm.h
+++ b/sound/virtio/virtio_pcm.h
@@ -53,6 +53,7 @@
  *        substream operators, including @stopped.
  * @buffer_bytes: Current buffer size in bytes.
  * @hw_ptr: Substream hardware pointer value in bytes [0 ... buffer_bytes).
+ * @playback_latency_bytes: Device delay cached independently of the ALSA runtime.
  * @capture_ready: Managed-mode bytes copied but not yet acknowledged by ALSA;
  *                 always zero in free-running mode.
  * @capture_boundary: ALSA boundary captured for this prepared epoch.
@@ -113,6 +114,7 @@
 	bool resetting;
 	size_t buffer_bytes;
 	size_t hw_ptr;
+	u32 playback_latency_bytes;
 	size_t capture_ready;
 	snd_pcm_uframes_t capture_boundary;
 	u64 capture_copied_total;
diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c
--- a/sound/virtio/virtio_pcm_msg.c
+++ b/sound/virtio/virtio_pcm_msg.c
@@ -1002,12 +1002,10 @@
 	/* Keep latched errors until prepare establishes a fresh epoch. */
 	vss->msg_count--;
 
-	if (vss->xfer_enabled) {
-		struct snd_pcm_runtime *runtime = vss->substream->runtime;
-
-		runtime->delay = bytes_to_frames(runtime, latency_bytes);
+	/* A queued request can outlive the ALSA runtime after hw_free. */
+	vss->playback_latency_bytes = latency_bytes;
+	if (vss->prepared && !vss->preparing)
 		virtsnd_pcm_schedule_period_elapsed(vss);
-	}
 	if (!vss->msg_count)
 		wake_up_all(&vss->msg_empty);
 }
diff --git a/sound/virtio/virtio_pcm_ops.c b/sound/virtio/virtio_pcm_ops.c
--- a/sound/virtio/virtio_pcm_ops.c
+++ b/sound/virtio/virtio_pcm_ops.c
@@ -770,6 +770,12 @@
 		memset(&vss->pcm_indirect, 0, sizeof(vss->pcm_indirect));
 		vss->pcm_indirect.sw_buffer_size = vss->buffer_bytes;
 		vss->pcm_indirect.hw_buffer_size = vss->buffer_bytes;
+		if (vss->direction == SNDRV_PCM_STREAM_PLAYBACK) {
+			/* Keep unobserved completions below one physical ring. */
+			vss->pcm_indirect.hw_queue_size = vss->buffer_bytes -
+				snd_pcm_lib_period_bytes(substream);
+			vss->playback_latency_bytes = 0;
+		}
 		if (vss->direction == SNDRV_PCM_STREAM_CAPTURE) {
 			vss->capture_boundary = substream->runtime->boundary;
 			rc = virtsnd_pcm_capture_refresh_mode_locked(vss,
@@ -1143,6 +1149,14 @@
 	return rc;
 }
 
+/* The queue capacity keeps equal cursors unambiguous for playback. */
+static bool
+virtsnd_pcm_playback_progress_pending(struct virtio_pcm_substream *vss)
+{
+	guard(spinlock_irqsave)(&vss->lock);
+	return vss->xfer_xrun || vss->hw_ptr != vss->pcm_indirect.hw_io;
+}
+
 /**
  * virtsnd_pcm_trigger_inner() - Process command for the PCM substream.
  * @substream: Kernel ALSA substream.
@@ -1197,7 +1211,8 @@
 		}
 
 		if (running) {
-			if (vss->direction == SNDRV_PCM_STREAM_CAPTURE)
+			if (vss->direction == SNDRV_PCM_STREAM_CAPTURE ||
+			    virtsnd_pcm_playback_progress_pending(vss))
 				virtsnd_pcm_schedule_period_elapsed(vss);
 			return 0;
 		}
@@ -1219,7 +1234,8 @@
 				return -ECANCELED;
 			vss->device_running = true;
 		}
-		if (!rc && (vss->direction == SNDRV_PCM_STREAM_CAPTURE))
+		if (!rc && (vss->direction == SNDRV_PCM_STREAM_CAPTURE ||
+			    virtsnd_pcm_playback_progress_pending(vss)))
 			virtsnd_pcm_schedule_period_elapsed(vss);
 		return rc;
 	case SNDRV_PCM_TRIGGER_SUSPEND:
@@ -1260,7 +1276,8 @@
 				virtsnd_pcm_trigger_failed(vss, epoch);
 			} else if (virtsnd_pcm_start_queue(vss, epoch)) {
 				virtsnd_pcm_trigger_failed(vss, epoch);
-			} else if (vss->direction == SNDRV_PCM_STREAM_CAPTURE) {
+			} else if (vss->direction == SNDRV_PCM_STREAM_CAPTURE ||
+				   virtsnd_pcm_playback_progress_pending(vss)) {
 				virtsnd_pcm_schedule_period_elapsed(vss);
 			}
 		}
@@ -1610,6 +1627,7 @@
 		}
 		if (!paused_pointer)
 			hw_ptr = vss->hw_ptr;
+		runtime->delay = bytes_to_frames(runtime, vss->playback_latency_bytes);
 	}
 	if (paused_pointer) {
 		if (defer_work)
@@ -2226,6 +2244,7 @@
 	vss->pcm_indirect.hw_data = position;
 	vss->pcm_indirect.sw_io = position;
 	vss->pcm_indirect.sw_data = position;
+	vss->playback_latency_bytes = 0;
 	vss->pcm_indirect.hw_ready = 0;
 	vss->pcm_indirect.sw_ready = 0;
 	vss->pcm_indirect.appl_ptr = runtime->status->hw_ptr;