[ 3.5.y.z extended stable ] Patch "ALSA: usb-audio: do not trust too-big wMaxPacketSize values" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 27efaa51591e9659fa534bf307d10d59b5f1ee9d Mon Sep 17 00:00:00 2001
From: Clemens Ladisch <clemens at ladisch.de>
Date: Thu, 8 Aug 2013 11:24:55 +0200
Subject: [PATCH] ALSA: usb-audio: do not trust too-big wMaxPacketSize values

commit 57e6dae1087bbaa6b33d3dd8a8e90b63888939a3 upstream.

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 at gmail.com>
Tested-by: James Stone <jamesmstone at gmail.com>
Signed-off-by: Clemens Ladisch <clemens at ladisch.de>
Signed-off-by: Takashi Iwai <tiwai at suse.de>
[ luis: backported to 3.5: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 sound/usb/endpoint.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index 060dccb..5fa3bd7 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -582,17 +582,16 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep,
 	ep->stride = frame_bits >> 3;
 	ep->silence_value = 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)
--
1.8.3.2