linux-kernel - PROBLEM: USB ACM device does not work

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Message-Id: <200906282229.35713.ars3n@yandex.ru>
Date:	Sun, 28 Jun 2009 22:29:26 +0400
From:	Arseniy Lartsev <ars3n@...dex.ru>
To:	linux-kernel@...r.kernel.org
Subject: PROBLEM: USB ACM device does not work

I've got a piece of hardware (this one:
http://masterkit.ru/main/set.php?num=1153) which acts like an USB ACM
device and is handeled by the cdc_acm module. /dev/ttyACM0 does appear
but any attempt to write something to the device with a simple
"echo something >/dev/ttyACM0" fails with "Invalid argument" error.

The reason is that the driver calls usb_submit_urb in acm_start_wb()
but doesn't set interval in the urb structure, so it remains zero and
causes usb_submit_urb to return -EINVAL. Reading from device also does
not work due to the same problem in acm_rx_tasklet().

After setting urb interval to 128, this particular device works fine
for me (though I'm not sure that it's a right solution in general).

Corresponding patch (against 2.6.30) is:

--- linux-2.6.30.orig/drivers/usb/class/cdc-acm.c
+++ linux-2.6.30/drivers/usb/class/cdc-acm.c
@@ -182,6 +182,7 @@ static int acm_start_wb(struct acm *acm,
 	wb->urb->transfer_dma = wb->dmah;
 	wb->urb->transfer_buffer_length = wb->len;
 	wb->urb->dev = acm->dev;
+	wb->urb->interval = 128;

 	if ((rc = usb_submit_urb(wb->urb, GFP_ATOMIC)) < 0) {
 		dbg("usb_submit_urb(write bulk) failed: %d", rc);
@@ -453,6 +454,7 @@ urbs:
 				  acm_read_bulk, rcv);
 		rcv->urb->transfer_dma = buf->dma;
 		rcv->urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
+		rcv->urb->interval = 128;

 		/* This shouldn't kill the driver as unsuccessful URBs are returned to the
 		   free-urbs-pool and resubmited ASAP */

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