[alsa-devel] [PATCH] 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
sampling frequency while the interface is active.

Reset the interface after setting the sampling frequency, 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 file, causing distorted sound
on sample rate changes.

Signed-off-by: Torstein Hegge <hegge at resisty.net>
---
 sound/usb/pcm.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index d82e378..01978a6 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -710,6 +710,16 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
 		subs->need_setup_ep = false;
 	}
 
+	/* Some devices doesn't respond to sample rate changes while the
+	 * interface is active. */
+	switch (subs->stream->chip->usb_id) {
+	case USB_ID(0x0d8c, 0x0304): /* C-Media - Schiit USB Interface */
+	case USB_ID(0x0d8c, 0x0309): /* C-Media CM6631 */
+		usb_set_interface(subs->dev, subs->cur_audiofmt->iface,
+					subs->cur_audiofmt->altset_idx);
+		break;
+	}
+
 	/* some unit conversions in runtime */
 	subs->data_endpoint->maxframesize =
 		bytes_to_frames(runtime, subs->data_endpoint->maxpacksize);
-- 
1.7.10.4