ALSA: pcm: fix playback silence - correct incremental silencing

Commit 9a826dd ("[ALSA] pcm core: fix silence_start calculations")
came with exactly the right commit message, but the patch just made
things broken in a different way: We'd fill at a too low address if the
area was already partially zeroed, so we'd under-fill. This affected
both thresholded mode (where it was somewhat less likely) and top-up
mode (where it would be the case consistently).

Co-developed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230505155244.2312199-3-oswald.buddenhagen@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: perexg

sound/core/pcm_lib.c

@@ -87,9 +87,7 @@ void snd_pcm_playback_silence(struct snd_pcm_substream *substream, snd_pcm_ufram
 			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;
+			runtime->silence_start = runtime->status->hw_ptr;
 		} else {
 			ofs = runtime->status->hw_ptr;
 			frames = new_hw_ptr - ofs;
@@ -98,18 +96,16 @@ void snd_pcm_playback_silence(struct snd_pcm_substream *substream, snd_pcm_ufram
 			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;
 			}
+			runtime->silence_start = new_hw_ptr;
 		}
 		frames = runtime->buffer_size - runtime->silence_filled;
 	}
 	if (snd_BUG_ON(frames > runtime->buffer_size))
 		return;
 	if (frames == 0)
 		return;
-	ofs = runtime->silence_start % runtime->buffer_size;
+	ofs = (runtime->silence_start + runtime->silence_filled) % runtime->buffer_size;
 	while (frames > 0) {
 		transfer = ofs + frames > runtime->buffer_size ? runtime->buffer_size - ofs : frames;
 		err = fill_silence_frames(substream, ofs, transfer);