ALSA: pcm: Revert "ALSA: pcm: rewrite snd_pcm_playback_silence()"

This reverts commit 9f656705c5faa18afb26d922cfc64f9fd103c38d.

There was a regression (in the top-up mode). Unfortunately, the patch
provided from the author of this commit is not easy to review.

Keep the updated and new comments in headers.
Also add a new comment that documents the missed API constraint which
led to the regression.

Reported-by: Jeff Chua <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jaroslav Kysela <[e-mail]>
Signed-off-by: Oswald Buddenhagen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/core/pcm_lib.c

···
  *
  * when runtime->silence_size >= runtime->boundary - fill processed area with silence immediately
  */
-void snd_pcm_playback_silence(struct snd_pcm_substream *substream)
+void snd_pcm_playback_silence(struct snd_pcm_substream *substream, snd_pcm_uframes_t new_hw_ptr)
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
-	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;
+	snd_pcm_uframes_t frames, 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) {
-		frames = runtime->silence_threshold - noise_dist;
-		if (frames <= 0)
+		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 > runtime->silence_size)
 			frames = runtime->silence_size;
 	} else {
-		frames = runtime->buffer_size - noise_dist;
-		if (frames <= 0)
-			return;
+		/*
+		 * This filling mode aims at free-running mode (used for example by dmix),
+		 * which doesn't update the application pointer.
+		 */
+		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;
 	}
-
 	if (snd_BUG_ON(frames > runtime->buffer_size))
 		return;
-	ofs = (runtime->silence_start + runtime->silence_filled) % runtime->buffer_size;
-	do {
+	if (frames == 0)
+		return;
+	ofs = runtime->silence_start % runtime->buffer_size;
+	while (frames > 0) {
 		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);
 }
 
···
 		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)
···
 		snd_BUG_ON(crossed_boundary != 1);
 		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);
 

sound/core/pcm_local.h

···
 			 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);
+void snd_pcm_playback_silence(struct snd_pcm_substream *substream,
+			      snd_pcm_uframes_t new_hw_ptr);
 
 static inline snd_pcm_uframes_t
 snd_pcm_avail(struct snd_pcm_substream *substream)

sound/core/pcm_native.c

···
 	if (snd_pcm_running(substream)) {
 		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 		    runtime->silence_size > 0)
-			snd_pcm_playback_silence(substream);
+			snd_pcm_playback_silence(substream, ULONG_MAX);
 		err = snd_pcm_update_state(substream, runtime);
 	}
 	snd_pcm_stream_unlock_irq(substream);
···
 	__snd_pcm_set_state(runtime, state);
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 	    runtime->silence_size > 0)
-		snd_pcm_playback_silence(substream);
+		snd_pcm_playback_silence(substream, ULONG_MAX);
 	snd_pcm_timer_notify(substream, SNDRV_TIMER_EVENT_MSTART);
 }
 
···
 	runtime->control->appl_ptr = runtime->status->hw_ptr;
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 	    runtime->silence_size > 0)
-		snd_pcm_playback_silence(substream);
+		snd_pcm_playback_silence(substream, ULONG_MAX);
 	snd_pcm_stream_unlock_irq(substream);
 }