ALSA: pcm: rewrite snd_pcm_playback_silence()

The auto-silencer supports two modes: "thresholded" to fill up "just
enough", and "top-up" to fill up "as much as possible". The two modes
used rather distinct code paths, which this patch unifies. The only
remaining distinction is how much we actually want to fill.

This fixes a bug in thresholded mode, where we failed to use new_hw_ptr,
resulting in under-fill.

Top-up mode is now more well-behaved and much easier to understand in
corner cases.

This also updates comments in the proximity of silencing-related data
structures.
Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230420113324.877164-1-oswald.buddenhagen@gmx.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

@@ -1577,14 +1577,19 @@ are the contents of this file:
           unsigned int period_step;
           unsigned int sleep_min;		/* min ticks to sleep */
           snd_pcm_uframes_t start_threshold;
-          snd_pcm_uframes_t stop_threshold;
-          snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
-                                                  noise is nearest than this */
-          snd_pcm_uframes_t silence_size;	/* Silence filling size */
+          /*
+           * The following two thresholds alleviate playback buffer underruns; when
+           * hw_avail drops below the threshold, the respective action is triggered:
+           */
+          snd_pcm_uframes_t stop_threshold;	/* - stop playback */
+          snd_pcm_uframes_t silence_threshold;	/* - pre-fill buffer with silence */
+          snd_pcm_uframes_t silence_size;       /* max size of silence pre-fill; when >= boundary,
+                                                 * fill played area with silence immediately */
           snd_pcm_uframes_t boundary;	/* pointers wrap point */
   
-          snd_pcm_uframes_t silenced_start;
-          snd_pcm_uframes_t silenced_size;
+          /* internal data of auto-silencer */
+          snd_pcm_uframes_t silence_start; /* starting pointer to silence area */
+          snd_pcm_uframes_t silence_filled; /* size filled with silence */
   
           snd_pcm_sync_id_t sync;		/* hardware synchronization ID */
   

include/sound/pcm.h

@@ -378,18 +378,18 @@ struct snd_pcm_runtime {
 	unsigned int rate_den;
 	unsigned int no_period_wakeup: 1;
 
-	/* -- SW params -- */
-	int tstamp_mode;		/* mmap timestamp is updated */
+	/* -- SW params; see struct snd_pcm_sw_params for comments -- */
+	int tstamp_mode;
   	unsigned int period_step;
 	snd_pcm_uframes_t start_threshold;
 	snd_pcm_uframes_t stop_threshold;
-	snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
-						noise is nearest than this */
-	snd_pcm_uframes_t silence_size;	/* Silence filling size */
-	snd_pcm_uframes_t boundary;	/* pointers wrap point */
+	snd_pcm_uframes_t silence_threshold;
+	snd_pcm_uframes_t silence_size;
+	snd_pcm_uframes_t boundary;
 
+	/* internal data of auto-silencer */
 	snd_pcm_uframes_t silence_start; /* starting pointer to silence area */
-	snd_pcm_uframes_t silence_filled; /* size filled with silence */
+	snd_pcm_uframes_t silence_filled; /* already filled part of silence area */
 
 	union snd_pcm_sync_id sync;	/* hardware synchronization ID */
 

include/uapi/sound/asound.h

@@ -429,9 +429,14 @@ struct snd_pcm_sw_params {
 	snd_pcm_uframes_t avail_min;		/* min avail frames for wakeup */
 	snd_pcm_uframes_t xfer_align;		/* obsolete: xfer size need to be a multiple */
 	snd_pcm_uframes_t start_threshold;	/* min hw_avail frames for automatic start */
-	snd_pcm_uframes_t stop_threshold;	/* min avail frames for automatic stop */
-	snd_pcm_uframes_t silence_threshold;	/* min distance from noise for silence filling */
-	snd_pcm_uframes_t silence_size;		/* silence block size */
+	/*
+	 * The following two thresholds alleviate playback buffer underruns; when
+	 * hw_avail drops below the threshold, the respective action is triggered:
+	 */
+	snd_pcm_uframes_t stop_threshold;	/* - stop playback */
+	snd_pcm_uframes_t silence_threshold;	/* - pre-fill buffer with silence */
+	snd_pcm_uframes_t silence_size;		/* max size of silence pre-fill; when >= boundary,
+						 * fill played area with silence immediately */
 	snd_pcm_uframes_t boundary;		/* pointers wrap point */
 	unsigned int proto;			/* protocol version */
 	unsigned int tstamp_type;		/* timestamp type (req. proto >= 2.0.12) */

sound/core/pcm_lib.c

@@ -42,70 +42,56 @@ static int fill_silence_frames(struct snd_pcm_substream *substream,
  *
  * when runtime->silence_size >= runtime->boundary - fill processed area with silence immediately
  */
-void snd_pcm_playback_silence(struct snd_pcm_substream *substream, snd_pcm_uframes_t new_hw_ptr)
+void snd_pcm_playback_silence(struct snd_pcm_substream *substream)
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
-	snd_pcm_uframes_t frames, ofs, transfer;
+	snd_pcm_uframes_t appl_ptr = READ_ONCE(runtime->control->appl_ptr);
+	snd_pcm_sframes_t added, hw_avail, frames;
+	snd_pcm_uframes_t noise_dist, ofs, transfer;
 	int err;
 
+	added = appl_ptr - runtime->silence_start;
+	if (added) {
+		if (added < 0)
+			added += runtime->boundary;
+		if (added < runtime->silence_filled)
+			runtime->silence_filled -= added;
+		else
+			runtime->silence_filled = 0;
+		runtime->silence_start = appl_ptr;
+	}
+
+	// This will "legitimately" turn negative on underrun, and will be mangled
+	// into a huge number by the boundary crossing handling. The initial state
+	// might also be not quite sane. The code below MUST account for these cases.
+	hw_avail = appl_ptr - runtime->status->hw_ptr;
+	if (hw_avail < 0)
+		hw_avail += runtime->boundary;
+
+	noise_dist = hw_avail + runtime->silence_filled;
 	if (runtime->silence_size < runtime->boundary) {
-		snd_pcm_sframes_t noise_dist, n;
-		snd_pcm_uframes_t appl_ptr = READ_ONCE(runtime->control->appl_ptr);
-		if (runtime->silence_start != appl_ptr) {
-			n = appl_ptr - runtime->silence_start;
-			if (n < 0)
-				n += runtime->boundary;
-			if ((snd_pcm_uframes_t)n < runtime->silence_filled)
-				runtime->silence_filled -= n;
-			else
-				runtime->silence_filled = 0;
-			runtime->silence_start = appl_ptr;
-		}
-		if (runtime->silence_filled >= runtime->buffer_size)
-			return;
-		noise_dist = snd_pcm_playback_hw_avail(runtime) + runtime->silence_filled;
-		if (noise_dist >= (snd_pcm_sframes_t) runtime->silence_threshold)
-			return;
 		frames = runtime->silence_threshold - noise_dist;
+		if (frames <= 0)
+			return;
 		if (frames > runtime->silence_size)
 			frames = runtime->silence_size;
 	} else {
-		if (new_hw_ptr == ULONG_MAX) {	/* initialization */
-			snd_pcm_sframes_t avail = snd_pcm_playback_hw_avail(runtime);
-			if (avail > runtime->buffer_size)
-				avail = runtime->buffer_size;
-			runtime->silence_filled = avail > 0 ? avail : 0;
-			runtime->silence_start = (runtime->status->hw_ptr +
-						  runtime->silence_filled) %
-						 runtime->boundary;
-		} else {
-			ofs = runtime->status->hw_ptr;
-			frames = new_hw_ptr - ofs;
-			if ((snd_pcm_sframes_t)frames < 0)
-				frames += runtime->boundary;
-			runtime->silence_filled -= frames;
-			if ((snd_pcm_sframes_t)runtime->silence_filled < 0) {
-				runtime->silence_filled = 0;
-				runtime->silence_start = new_hw_ptr;
-			} else {
-				runtime->silence_start = ofs;
-			}
-		}
-		frames = runtime->buffer_size - runtime->silence_filled;
+		frames = runtime->buffer_size - noise_dist;
+		if (frames <= 0)
+			return;
 	}
+
 	if (snd_BUG_ON(frames > runtime->buffer_size))
 		return;
-	if (frames == 0)
-		return;
-	ofs = runtime->silence_start % runtime->buffer_size;
-	while (frames > 0) {
+	ofs = (runtime->silence_start + runtime->silence_filled) % runtime->buffer_size;
+	do {
 		transfer = ofs + frames > runtime->buffer_size ? runtime->buffer_size - ofs : frames;
 		err = fill_silence_frames(substream, ofs, transfer);
 		snd_BUG_ON(err < 0);
 		runtime->silence_filled += transfer;
 		frames -= transfer;
 		ofs = 0;
-	}
+	} while (frames > 0);
 	snd_pcm_dma_buffer_sync(substream, SNDRV_DMA_SYNC_DEVICE);
 }
 
@@ -439,10 +425,6 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
 		return 0;
 	}
 
-	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
-	    runtime->silence_size > 0)
-		snd_pcm_playback_silence(substream, new_hw_ptr);
-
 	if (in_interrupt) {
 		delta = new_hw_ptr - runtime->hw_ptr_interrupt;
 		if (delta < 0)
@@ -460,6 +442,10 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
 		runtime->hw_ptr_wrap += runtime->boundary;
 	}
 
+	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
+	    runtime->silence_size > 0)
+		snd_pcm_playback_silence(substream);
+
 	update_audio_tstamp(substream, &curr_tstamp, &audio_tstamp);
 
 	return snd_pcm_update_state(substream, runtime);

sound/core/pcm_local.h

@@ -29,8 +29,7 @@ int snd_pcm_update_state(struct snd_pcm_substream *substream,
 			 struct snd_pcm_runtime *runtime);
 int snd_pcm_update_hw_ptr(struct snd_pcm_substream *substream);
 
-void snd_pcm_playback_silence(struct snd_pcm_substream *substream,
-			      snd_pcm_uframes_t new_hw_ptr);
+void snd_pcm_playback_silence(struct snd_pcm_substream *substream);
 
 static inline snd_pcm_uframes_t
 snd_pcm_avail(struct snd_pcm_substream *substream)

sound/core/pcm_native.c

@@ -958,7 +958,7 @@ static int snd_pcm_sw_params(struct snd_pcm_substream *substream,
 	if (snd_pcm_running(substream)) {
 		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 		    runtime->silence_size > 0)
-			snd_pcm_playback_silence(substream, ULONG_MAX);
+			snd_pcm_playback_silence(substream);
 		err = snd_pcm_update_state(substream, runtime);
 	}
 	snd_pcm_stream_unlock_irq(substream);
@@ -1455,7 +1455,7 @@ static void snd_pcm_post_start(struct snd_pcm_substream *substream,
 	__snd_pcm_set_state(runtime, state);
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 	    runtime->silence_size > 0)
-		snd_pcm_playback_silence(substream, ULONG_MAX);
+		snd_pcm_playback_silence(substream);
 	snd_pcm_timer_notify(substream, SNDRV_TIMER_EVENT_MSTART);
 }
 
@@ -1916,7 +1916,7 @@ static void snd_pcm_post_reset(struct snd_pcm_substream *substream,
 	runtime->control->appl_ptr = runtime->status->hw_ptr;
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 	    runtime->silence_size > 0)
-		snd_pcm_playback_silence(substream, ULONG_MAX);
+		snd_pcm_playback_silence(substream);
 	snd_pcm_stream_unlock_irq(substream);
 }