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

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Message-ID: <Pine.LNX.4.44L0.1110151459030.15129-100000@netrider.rowland.org>
Date:	Sat, 15 Oct 2011 15:04:28 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Johannes Stezenbach <js@...21.net>
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, 15 Oct 2011, Johannes Stezenbach wrote:

> What I meant to say is Markus' statement that the device only
> works at a certain transfer size cannot be true since
> this size is not visible to the device via the USB bus.

That's what I would expect, too.  But did you take a look at the usbmon 
traces Markus acquired?

	http://marc.info/?l=linux-usb&m=131845614819045&w=2

They aren't completely definitive because the communications between
the computer and the device _before_ the bulk transfers started were
different.  However they do clearly show the device working with
24064-byte transfers and not working with 12288+11776-byte transfers.

> If you queue two URBs, one 12288 and 11776 bytes, the device
> does not see any difference to one URB with 24064.  It's just not
> in the USB wire protocol.

You can argue until you're blue in the face.  It won't affect the 
results that Markus got in real life.

>  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.

Alan Stern

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