ALSA: usb-audio: do not trust too-big wMaxPacketSize values

The driver used to assume that the streaming endpoint's wMaxPacketSize
value would be an indication of how much data the endpoint expects or
sends, and compute the number of packets per URB using this value.

However, the Focusrite Scarlett 2i4 declares a value of 1024 bytes,
while only about 88 or 44 bytes are be actually used.  This discrepancy
would result in URBs with far too few packets, which would not work
correctly on the EHCI driver.

To get correct URBs, use wMaxPacketSize only as an upper limit on the
packet size.

Reported-by: James Stone <jamesmstone@gmail.com>
Tested-by: James Stone <jamesmstone@gmail.com>
Cc: <stable@vger.kernel.org> # 2.6.35+
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 7a444b5501d91d28a4e7e8fb5e044c2b4e7ea41b..659950e5b94f6b411577a84821f1a1c4d93b4593 100644 (file)
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -591,17 +591,16 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep,
        ep->stride = frame_bits >> 3;
        ep->silence_value = pcm_format == SNDRV_PCM_FORMAT_U8 ? 0x80 : 0;
 
-       /* calculate max. frequency */
-       if (ep->maxpacksize) {
+       /* assume max. frequency is 25% higher than nominal */
+       ep->freqmax = ep->freqn + (ep->freqn >> 2);
+       maxsize = ((ep->freqmax + 0xffff) * (frame_bits >> 3))
+                               >> (16 - ep->datainterval);
+       /* but wMaxPacketSize might reduce this */
+       if (ep->maxpacksize && ep->maxpacksize < maxsize) {
                /* whatever fits into a max. size packet */
                maxsize = ep->maxpacksize;
                ep->freqmax = (maxsize / (frame_bits >> 3))
                                << (16 - ep->datainterval);
-       } else {
-               /* no max. packet size: just take 25% higher than nominal */
-               ep->freqmax = ep->freqn + (ep->freqn >> 2);
-               maxsize = ((ep->freqmax + 0xffff) * (frame_bits >> 3))
-                               >> (16 - ep->datainterval);
        }
 
        if (ep->fill_max)