linux-kernel - Re: [Patch] Increase USBFS Bulk Transfer size

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Message-ID: <20111017181117.GA17422@sig21.net>
Date:	Mon, 17 Oct 2011 20:11:18 +0200
From:	Johannes Stezenbach <js@...21.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Markus Rechberger <mrechberger@...il.com>,
	Greg KH <gregkh@...e.de>, USB list <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Increase USBFS Bulk Transfer size

On Sat, Oct 15, 2011 at 03:04:28PM -0400, Alan Stern wrote:
> On Sat, 15 Oct 2011, Johannes Stezenbach wrote:
> 
> >  It would make a difference if the
> > device violated the spec and sent 188 byte packets. However, the
> > spec says a short packet terminates the transfer.  But I wonder
> > if this is really the case?
> 
> The device does not send short packets.  If it did, the 24064-byte 
> transfers would end early.

Re-reading the USB-2.0 standard, a short packet which terminates
the transfer is defined by packet_size < wMaxPacketSize,
not by packet_size < 512.
Thus wMaxPacketSize == 188 (or 2*188) might be possible.

There is a comment in linux/drivers/usb/host/ehci-q.c:

	/* The USB spec says that high speed bulk endpoints
	 * always use 512 byte maxpacket.  But some device
	 * vendors decided to ignore that, and MSFT is happy
	 * to help them do so.  So now people expect to use
	 * such nonconformant devices with Linux too; sigh.
	 */

Maybe we should look at the descriptors?


Johannes
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