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>

1 files changed, 11 insertions, 0 deletions

diff --git a/sound/usb/quirks.c b/sound/usb/quirks.c
index e71fc8bc3a70..2c971858d6b7 100644
--- a/sound/usb/quirks.c
+++ b/sound/usb/quirks.c
@@ -861,6 +861,17 @@ void snd_usb_endpoint_start_quirk(struct snd_usb_endpoint *ep)
         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,