ALSA: usb-audio: automatically detect feedback format

There are two USB Audio Class specifications (v1 and v2), but neither of
them clearly defines the feedback format for high-speed UAC v1 devices.
Add to this whatever the Creative and M-Audio firmware writers have been
smoking, and it becomes impossible to predict the exact feedback format
used by a particular device.

Therefore, automatically detect the feedback format by looking at the
magnitude of the first received feedback value.

Also, this allows us to get rid of some special cases for E-Mu devices.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -237,6 +237,7 @@ static int set_format(struct snd_usb_substream *subs, struct audioformat *fmt)
 	subs->datainterval = fmt->datainterval;
 	subs->syncpipe = subs->syncinterval = 0;
 	subs->maxpacksize = fmt->maxpacksize;
+	subs->syncmaxsize = 0;
 	subs->fill_max = 0;
 
 	/* we need a sync pipe in async OUT or adaptive IN mode */
@@ -283,6 +284,7 @@ static int set_format(struct snd_usb_substream *subs, struct audioformat *fmt)
 			subs->syncinterval = get_endpoint(alts, 1)->bInterval - 1;
 		else
 			subs->syncinterval = 3;
+		subs->syncmaxsize = le16_to_cpu(get_endpoint(alts, 1)->wMaxPacketSize);
 	}
 
 	/* always fill max packet size */