[PATCH] [RFC] alsaloop: add feedback frequency control support for UAC2 gadgets

[Ruslan Bilovol <ruslan.bilovol@xxxxxxxxx>]

Add support of special "PCM Feedback Frequency Hz"
UAC2 Gadget mixer control that is designed to notify
host about real sampling frequency of the gadget so
it can adjust number of samples that hosts sends to
the gadget.

This is useful if both host and gadget has its own
internal freerunning clock, so host can adjust
number of samples sent, preventing overrun/underrun
conditions.

This patch reuses logic of the "PCM Rate Shift 100000"
control used in case of in-kernel ALSA loopback
driver. The only difference is alsaloop reports not
rate shift but frequency in Hz

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@xxxxxxxxx>
---
 alsaloop/alsaloop.h |  1 +
 alsaloop/pcmjob.c   | 35 +++++++++++++++++++++++++----------
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/alsaloop/alsaloop.h b/alsaloop/alsaloop.h
index c4aa618..9a50a42 100644
--- a/alsaloop/alsaloop.h
+++ b/alsaloop/alsaloop.h
@@ -122,6 +122,7 @@ struct loopback_handle {
 	unsigned int ctl_pollfd_count;
 	snd_ctl_elem_value_t *ctl_notify;
 	snd_ctl_elem_value_t *ctl_rate_shift;
+	snd_ctl_elem_value_t *ctl_fback_freq;
 	snd_ctl_elem_value_t *ctl_active;
 	snd_ctl_elem_value_t *ctl_format;
 	snd_ctl_elem_value_t *ctl_rate;
diff --git a/alsaloop/pcmjob.c b/alsaloop/pcmjob.c
index 6a9aff4..b3802a8 100644
--- a/alsaloop/pcmjob.c
+++ b/alsaloop/pcmjob.c
@@ -1058,15 +1058,22 @@ static int set_notify(struct loopback_handle *lhandle, int enable)
 
 static int set_rate_shift(struct loopback_handle *lhandle, double pitch)
 {
-	int err;
+	int err = 0;
 
-	if (lhandle->ctl_rate_shift == NULL)
-		return 0;
-	snd_ctl_elem_value_set_integer(lhandle->ctl_rate_shift, 0, pitch * 100000);
-	err = snd_ctl_elem_write(lhandle->ctl, lhandle->ctl_rate_shift);
-	if (err < 0) {
-		logit(LOG_CRIT, "Cannot set PCM Rate Shift element for %s: %s\n", lhandle->id, snd_strerror(err));
-		return err;
+	if (lhandle->ctl_rate_shift) {
+		snd_ctl_elem_value_set_integer(lhandle->ctl_rate_shift, 0, pitch * 100000);
+		err = snd_ctl_elem_write(lhandle->ctl, lhandle->ctl_rate_shift);
+		if (err < 0) {
+			logit(LOG_CRIT, "Cannot set PCM Rate Shift element for %s: %s\n", lhandle->id, snd_strerror(err));
+			return err;
+		}
+	} else if (lhandle->ctl_fback_freq) {
+		snd_ctl_elem_value_set_integer(lhandle->ctl_fback_freq, 0, lhandle->rate * (2.0 - pitch));
+		err = snd_ctl_elem_write(lhandle->ctl, lhandle->ctl_fback_freq);
+		if (err < 0) {
+			logit(LOG_CRIT, "Cannot set PCM Feedback Frequency element for %s: %s\n", lhandle->id, snd_strerror(err));
+			return err;
+		}
 	}
 	return 0;
 }
@@ -1195,6 +1202,7 @@ static int openctl(struct loopback_handle *lhandle, int device, int subdevice)
 	int err;
 
 	lhandle->ctl_rate_shift = NULL;
+	lhandle->ctl_fback_freq = NULL;
 	if (lhandle->loopback->play == lhandle) {
 		if (lhandle->loopback->controls)
 			goto __events;
@@ -1204,6 +1212,8 @@ static int openctl(struct loopback_handle *lhandle, int device, int subdevice)
 			&lhandle->ctl_notify);
 	openctl_elem(lhandle, device, subdevice, "PCM Rate Shift 100000",
 			&lhandle->ctl_rate_shift);
+	openctl_elem(lhandle, device, subdevice, "PCM Feedback Frequency Hz",
+			&lhandle->ctl_fback_freq);
 	set_rate_shift(lhandle, 1);
 	openctl_elem(lhandle, device, subdevice, "PCM Slave Active",
 			&lhandle->ctl_active);
@@ -1289,6 +1299,9 @@ static int closeit(struct loopback_handle *lhandle)
 	if (lhandle->ctl_rate_shift)
 		snd_ctl_elem_value_free(lhandle->ctl_rate_shift);
 	lhandle->ctl_rate_shift = NULL;
+	if (lhandle->ctl_fback_freq)
+		snd_ctl_elem_value_free(lhandle->ctl_fback_freq);
+	lhandle->ctl_fback_freq = NULL;
 	if (lhandle->ctl)
 		err = snd_ctl_close(lhandle->ctl);
 	lhandle->ctl = NULL;
@@ -1334,9 +1347,11 @@ int pcmjob_init(struct loopback *loop)
 	snprintf(id, sizeof(id), "%s/%s", loop->play->id, loop->capt->id);
 	id[sizeof(id)-1] = '\0';
 	loop->id = strdup(id);
-	if (loop->sync == SYNC_TYPE_AUTO && loop->capt->ctl_rate_shift)
+	if (loop->sync == SYNC_TYPE_AUTO && (loop->capt->ctl_rate_shift ||
+			loop->capt->ctl_fback_freq))
 		loop->sync = SYNC_TYPE_CAPTRATESHIFT;
-	if (loop->sync == SYNC_TYPE_AUTO && loop->play->ctl_rate_shift)
+	if (loop->sync == SYNC_TYPE_AUTO && (loop->play->ctl_rate_shift ||
+			loop->play->ctl_fback_freq))
 		loop->sync = SYNC_TYPE_PLAYRATESHIFT;
 #ifdef USE_SAMPLERATE
 	if (loop->sync == SYNC_TYPE_AUTO && loop->src_enable)
-- 
1.9.1