[alsa-devel] [PATCH v2] ALSA: usb: Work around CM6631 sample rate change bug

[Torstein Hegge]

The C-Media CM6631 USB-to-S/PDIF receiver doesn't respond to changes in
sample rate while the interface is active.

Reset the interface after setting the sampling frequency on sample rate
changes, to ensure that the sample rate set by snd_usb_init_sample_rate() is
used. Otherwise, the device will try to use the sample rate of the previous
stream, causing distorted sound on sample rate changes.

Signed-off-by: Torstein Hegge <hegge at resisty.net>
---
 sound/usb/clock.c |   25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/sound/usb/clock.c b/sound/usb/clock.c
index 5e634a2..d12a9c4 100644
--- a/sound/usb/clock.c
+++ b/sound/usb/clock.c
@@ -255,6 +255,7 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 	unsigned char data[4];
 	int err, crate;
 	int clock = snd_usb_clock_find_source(chip, fmt->clock);
+	unsigned int previous_rate;
 
 	if (clock < 0)
 		return clock;
@@ -266,6 +267,18 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 		return -ENXIO;
 	}
 
+	if ((err = snd_usb_ctl_msg(dev, usb_rcvctrlpipe(dev, 0), UAC2_CS_CUR,
+				   USB_TYPE_CLASS | USB_RECIP_INTERFACE | USB_DIR_IN,
+				   UAC2_CS_CONTROL_SAM_FREQ << 8,
+				   snd_usb_ctrl_intf(chip) | (clock << 8),
+				   data, sizeof(data))) < 0) {
+		snd_printk(KERN_WARNING "%d:%d:%d: cannot get freq (v2)\n",
+			   dev->devnum, iface, fmt->altsetting);
+		return err;
+	}
+
+	previous_rate = data[0] | (data[1] << 8) | (data[2] << 16) | (data[3] << 24);
+
 	data[0] = rate;
 	data[1] = rate >> 8;
 	data[2] = rate >> 16;
@@ -294,6 +307,18 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 	if (crate != rate)
 		snd_printd(KERN_WARNING "current rate %d is different from the runtime rate %d\n", crate, rate);
 
+	/* Some devices doesn't respond to sample rate changes while the
+	 * interface is active. */
+	if (crate != previous_rate) {
+		switch (chip->usb_id) {
+		case USB_ID(0x0d8c, 0x0304): /* C-Media - Schiit USB */
+		case USB_ID(0x0d8c, 0x0309): /* C-Media CM6631 */
+			usb_set_interface(dev, iface, 0);
+			usb_set_interface(dev, iface, fmt->altsetting);
+			break;
+		}
+	}
+
 	return 0;
 }
 
-- 
1.7.10.4