pcm: hw: setup explicit silencing for snd_pcm_drain by default

Some applications may not alignt transfers to the period size
and also the driver developers may not follow the consequeces of the
access beyond valid samples in the playback DMA buffer.

To avoid clicks, fill a little silence at the end of the playback
ring buffer when snd_pcm_drain() is called.

Related: https://lore.kernel.org/alsa-devel/20230420113324.877164-2-oswald.buddenhagen@gmx.de/
Related: https://lore.kernel.org/alsa-devel/20230405201219.2197789-2-oswald.buddenhagen@gmx.de/
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

src/pcm/pcm.c

@@ -6167,6 +6167,25 @@ int snd_pcm_hw_params_get_min_align(const snd_pcm_hw_params_t *params, snd_pcm_u
 	return 0;
 }
 
+#ifndef DOXYGEN
+void snd_pcm_sw_params_current_no_lock(snd_pcm_t *pcm, snd_pcm_sw_params_t *params)
+{
+	params->proto = SNDRV_PCM_VERSION;
+	params->tstamp_mode = pcm->tstamp_mode;
+	params->tstamp_type = pcm->tstamp_type;
+	params->period_step = pcm->period_step;
+	params->sleep_min = 0;
+	params->avail_min = pcm->avail_min;
+	sw_set_period_event(params, pcm->period_event);
+	params->xfer_align = 1;
+	params->start_threshold = pcm->start_threshold;
+	params->stop_threshold = pcm->stop_threshold;
+	params->silence_threshold = pcm->silence_threshold;
+	params->silence_size = pcm->silence_size;
+	params->boundary = pcm->boundary;
+}
+#endif
+
 /**
  * \brief Return current software configuration for a PCM
  * \param pcm PCM handle
@@ -6183,19 +6202,7 @@ int snd_pcm_sw_params_current(snd_pcm_t *pcm, snd_pcm_sw_params_t *params)
 		return -EIO;
 	}
 	__snd_pcm_lock(pcm); /* forced lock due to pcm field changes */
-	params->proto = SNDRV_PCM_VERSION;
+	snd_pcm_sw_params_current_no_lock(pcm, params);
-	params->tstamp_mode = pcm->tstamp_mode;
-	params->tstamp_type = pcm->tstamp_type;
-	params->period_step = pcm->period_step;
-	params->sleep_min = 0;
-	params->avail_min = pcm->avail_min;
-	sw_set_period_event(params, pcm->period_event);
-	params->xfer_align = 1;
-	params->start_threshold = pcm->start_threshold;
-	params->stop_threshold = pcm->stop_threshold;
-	params->silence_threshold = pcm->silence_threshold;
-	params->silence_size = pcm->silence_size;
-	params->boundary = pcm->boundary;
 	__snd_pcm_unlock(pcm);
 	return 0;
 }

src/pcm/pcm_hw.c

@@ -98,6 +98,8 @@ typedef struct {
 	bool mmap_control_fallbacked;
 	struct snd_pcm_sync_ptr *sync_ptr;
 
+	bool prepare_reset_sw_params;
+
 	int period_event;
 	snd_timer_t *period_timer;
 	struct pollfd period_timer_pfd;
@@ -534,6 +536,7 @@ static int snd_pcm_hw_sw_params(snd_pcm_t *pcm, snd_pcm_sw_params_t * params)
 		SYSMSG("SNDRV_PCM_IOCTL_SW_PARAMS failed (%i)", err);
 		goto out;
 	}
+	hw->prepare_reset_sw_params = false;
 	if ((snd_pcm_tstamp_type_t) params->tstamp_type != pcm->tstamp_type) {
 		if (hw->version < SNDRV_PROTOCOL_VERSION(2, 0, 12)) {
 			int on = (snd_pcm_tstamp_type_t) params->tstamp_type ==
@@ -660,7 +663,18 @@ static int snd_pcm_hw_hwsync(snd_pcm_t *pcm)
 static int snd_pcm_hw_prepare(snd_pcm_t *pcm)
 {
 	snd_pcm_hw_t *hw = pcm->private_data;
+	snd_pcm_sw_params_t sw_params;
 	int fd = hw->fd, err;
+
+	if (hw->prepare_reset_sw_params) {
+		snd_pcm_sw_params_current_no_lock(pcm, &sw_params);
+		if (ioctl(hw->fd, SNDRV_PCM_IOCTL_SW_PARAMS, sw_params) < 0) {
+			err = -errno;
+			SYSMSG("SNDRV_PCM_IOCTL_SW_PARAMS failed (%i)", err);
+			return err;
+		}
+		hw->prepare_reset_sw_params = false;
+	}
 	if (ioctl(fd, SNDRV_PCM_IOCTL_PREPARE) < 0) {
 		err = -errno;
 		SYSMSG("SNDRV_PCM_IOCTL_PREPARE failed (%i)", err);
@@ -718,7 +732,40 @@ static int snd_pcm_hw_drop(snd_pcm_t *pcm)
 static int snd_pcm_hw_drain(snd_pcm_t *pcm)
 {
 	snd_pcm_hw_t *hw = pcm->private_data;
+	snd_pcm_sw_params_t sw_params;
+	snd_pcm_uframes_t silence_size;
 	int err;
+
+	if (pcm->stream != SND_PCM_STREAM_PLAYBACK)
+		goto __skip_silence;
+	/* compute end silence size, align to period size + extra time */
+	snd_pcm_sw_params_current_no_lock(pcm, &sw_params);
+	if ((pcm->boundary % pcm->period_size) == 0) {
+		silence_size = pcm->period_size - (*pcm->appl.ptr % pcm->period_size);
+		if (silence_size == pcm->period_size)
+			silence_size = 0;
+	} else {
+		/* it not not easy to compute the period crossing point
+		 * in this case because the period is not aligned to the boundary
+		 * - use the full range (one period) in this case
+		 */
+		silence_size = pcm->period_size;
+	}
+	silence_size += pcm->rate / 10;	/* 1/10th of second */
+	if (sw_params.silence_size < silence_size) {
+		/* fill the silence soon as possible (in the bellow ioctl
+		 * or the next period wake up)
+		 */
+		sw_params.silence_threshold = pcm->buffer_size;
+		sw_params.silence_size = silence_size;
+		if (ioctl(hw->fd, SNDRV_PCM_IOCTL_SW_PARAMS, &sw_params) < 0) {
+			err = -errno;
+			SYSMSG("SNDRV_PCM_IOCTL_SW_PARAMS failed (%i)", err);
+			return err;
+		}
+		hw->prepare_reset_sw_params = true;
+	}
+__skip_silence:
 	if (ioctl(hw->fd, SNDRV_PCM_IOCTL_DRAIN) < 0) {
 		err = -errno;
 		SYSMSG("SNDRV_PCM_IOCTL_DRAIN failed (%i)", err);

src/pcm/pcm_local.h

@@ -366,6 +366,8 @@ struct _snd_pcm {
 	snd1_pcm_hw_param_get_max
 #define snd_pcm_hw_param_name		\
 	snd1_pcm_hw_param_name
+#define snd_pcm_sw_params_current_no_lock \
+	snd1_pcm_sw_params_current_no_lock
 
 int snd_pcm_new(snd_pcm_t **pcmp, snd_pcm_type_t type, const char *name,
 		snd_pcm_stream_t stream, int mode);
@@ -390,6 +392,8 @@ void snd_pcm_mmap_appl_forward(snd_pcm_t *pcm, snd_pcm_uframes_t frames);
 void snd_pcm_mmap_hw_backward(snd_pcm_t *pcm, snd_pcm_uframes_t frames);
 void snd_pcm_mmap_hw_forward(snd_pcm_t *pcm, snd_pcm_uframes_t frames);
 
+void snd_pcm_sw_params_current_no_lock(snd_pcm_t *pcm, snd_pcm_sw_params_t *params);
+
 snd_pcm_sframes_t snd_pcm_mmap_writei(snd_pcm_t *pcm, const void *buffer, snd_pcm_uframes_t size);
 snd_pcm_sframes_t snd_pcm_mmap_readi(snd_pcm_t *pcm, void *buffer, snd_pcm_uframes_t size);
 snd_pcm_sframes_t snd_pcm_mmap_writen(snd_pcm_t *pcm, void **bufs, snd_pcm_uframes_t size);