[PATCH v4] ALSA: usb: Work around CM6631 sample rate change bug

[Torstein Hegge <hegge@resisty.net>]

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.

The reset is performed for all current C-Media UAC V2.0 devices, including
CM6610, CM6620 and CM6631, but has only been tested with CM6631.

Signed-off-by: Torstein Hegge <hegge@resisty.net>
---
Changes since v3:
- Use given target rate instead of the new rate read from device when checking
  if a reset is needed. Some devices (at least Schiit Bifrost) doesn't report a
  reliable sample rate back right after setting a new rate. This appears to
  fix the issues with v3 of this patch with the Schiit, as reported by Chris
  Hermansen in http://thread.gmane.org/gmane.linux.alsa.user/36935/focus=37284

Changes since v2:
- More USB ids covered.
- int crate -> int cur_rate
- unsigned int previous_rate -> int prev_rate

 sound/usb/clock.c |   50 ++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 46 insertions(+), 4 deletions(-)

diff --git a/sound/usb/clock.c b/sound/usb/clock.c
index 5e634a2..60c2775 100644
--- a/sound/usb/clock.c
+++ b/sound/usb/clock.c
@@ -253,7 +253,7 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 {
 	struct usb_device *dev = chip->dev;
 	unsigned char data[4];
-	int err, crate;
+	int err, cur_rate, prev_rate;
 	int clock = snd_usb_clock_find_source(chip, fmt->clock);
 
 	if (clock < 0)
@@ -266,6 +266,18 @@ 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;
+	}
+
+	prev_rate = data[0] | (data[1] << 8) | (data[2] << 16) | (data[3] << 24);
+
 	data[0] = rate;
 	data[1] = rate >> 8;
 	data[2] = rate >> 16;
@@ -290,9 +302,39 @@ static int set_sample_rate_v2(struct snd_usb_audio *chip, int iface,
 		return err;
 	}
 
-	crate = data[0] | (data[1] << 8) | (data[2] << 16) | (data[3] << 24);
-	if (crate != rate)
-		snd_printd(KERN_WARNING "current rate %d is different from the runtime rate %d\n", crate, rate);
+	cur_rate = data[0] | (data[1] << 8) | (data[2] << 16) | (data[3] << 24);
+	if (cur_rate != rate) {
+		snd_printd(KERN_WARNING
+			   "current rate %d is different from the runtime rate %d\n",
+			   cur_rate, rate);
+	}
+
+	/* Some devices doesn't respond to sample rate changes while the
+	 * interface is active. */
+	if (rate != prev_rate) {
+		switch (chip->usb_id) {
+		/* C-Media CM6610/CM6620/CM6631 */
+		case USB_ID(0x054c, 0x06cf): /* Sony */
+		case USB_ID(0x0b05, 0x17a8): /* Asus Xonar Essence One */
+		case USB_ID(0x0d8c, 0x0301):
+		case USB_ID(0x0d8c, 0x0302):
+		case USB_ID(0x0d8c, 0x0304): /* CM6631 (Schiit) */
+		case USB_ID(0x0d8c, 0x0305):
+		case USB_ID(0x0d8c, 0x0306):
+		case USB_ID(0x0d8c, 0x0309): /* CM6631 */
+		case USB_ID(0x0d8c, 0x0310):
+		case USB_ID(0x0d8c, 0x0311): /* CM6610A */
+		case USB_ID(0x0d8c, 0x0312): /* CM6620A */
+		case USB_ID(0x0d8c, 0x0313): /* CM6630A */
+		case USB_ID(0x0d8c, 0x0314): /* CM6631A */
+		case USB_ID(0x0d8c, 0x0315): /* CM6632A */
+		case USB_ID(0x0d8c, 0x0319): /* CM6631A */
+		case USB_ID(0x200c, 0x1030): /* Reloop */
+			usb_set_interface(dev, iface, 0);
+			usb_set_interface(dev, iface, fmt->altsetting);
+			break;
+		}
+	}
 
 	return 0;
 }
-- 
1.7.10.4