ALSA: usb-audio: M-Audio FT C400 skip packet quirk
Attain constant real-world latency by skipping 16 data packets.
The number of packets to be skipped was found by trial and error.
Signed-off-by: Eldad Zack <eldad@fogrefinery.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index e71fc8b..2c97185 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -861,6 +861,17 @@
if ((le16_to_cpu(ep->chip->dev->descriptor.idVendor) == 0x23ba) &&
ep->type == SND_USB_ENDPOINT_TYPE_SYNC)
ep->skip_packets = 4;
+
+ /*
+ * M-Audio Fast Track C400 - when packets are not skipped, real world
+ * latency varies by approx. +/- 50 frames (at 96KHz) each time the
+ * stream is (re)started. When skipping packets 16 at endpoint start
+ * up, the real world latency is stable within +/- 1 frame (also
+ * across power cycles).
+ */
+ if (ep->chip->usb_id == USB_ID(0x0763, 0x2030) &&
+ ep->type == SND_USB_ENDPOINT_TYPE_DATA)
+ ep->skip_packets = 16;
}
void snd_usb_ctl_msg_quirk(struct usb_device *dev, unsigned int pipe,