[PATCH] ALSA: usb-audio: add implicit feedback quirk for Yamaha P-125

[Arseniy Lartsev <arseniy@alumni.chalmers.se>]

This digital piano has audio playback function with implicit feedback
endpoint.

On this device in particular, if USB host sends samples slightly too fast due
to lack of synchronization (50% chance of that happening), playback will fail
after a few minutes. This patch fixes the problem.

The feedback endpoint is, in fact, very nearly standard-compliant, but
set_sync_endpoint function currently uses different logic for feedback
endpoint discovery - the patch also adds a comment to clarify this behaviour.

Signed-off-by: Arseniy Lartsev <arseniy@alumni.chalmers.se>
---
 sound/usb/pcm.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 9c8930b..47ccea6 100644
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -351,6 +351,10 @@ static int set_sync_ep_implicit_fb_quirk(struct 
snd_usb_substream *subs,
 	case USB_ID(0x0582, 0x01d8): /* BOSS Katana */
 		/* BOSS Katana amplifiers do not need quirks */
 		return 0;
+	case USB_ID(0x0499, 0x1718): /* Yamaha P-125 digital piano */
+		ep = 0x86;
+		ifnum = 2;
+		goto add_sync_ep_from_ifnum;
 	}
 
 	if (attr == USB_ENDPOINT_SYNC_ASYNC &&
@@ -427,6 +431,21 @@ static int set_sync_endpoint(struct snd_usb_substream 
*subs,
 	if (err > 0)
 		return 0;
 
+	/*
+	 * Actually, what USB 2.0 standard says is this:
+	 *
+	 * A feedback endpoint (explicit or implicit) needs to be associated with
+	 * one (or more) isochronous data endpoints to which it provides feedback
+	 * service. <...> The first data endpoint and the feedback endpoint must
+	 * have the same endpoint number (and opposite direction). This ensures
+	 * that a data endpoint can uniquely identify its feedback endpoint by
+	 * searching for the first feedback endpoint that has an endpoint number
+	 * equal or less than its own endpoint number.
+	 *
+	 * However, it looks like hardware vendors never follow this except by
+	 * accident, so we may as well give up on it too. Look for a feedback
+	 * endpoint as a second endpoint on the same interface.
+	 */
 	if (altsd->bNumEndpoints < 2)
 		return 0;
 
-- 
2.10.0