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

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Date:	Fri, 14 Oct 2011 16:19:34 -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 Fri, 14 Oct 2011, Johannes Stezenbach wrote:

> I don't really want to help Markus with his proprietary, binary-only
> userspace driver crap, but I wonder why nobody seems to remember
> how the USB protocol works on the wire?

I remember it perfectly well.

>  The transfer size is
> never seen by the device, thus it cannot matter if two small URBs
> or one large URB are queued.  What matters is the packet size.

That's what I have been saying.  Markus's experience contradicts this,
however.

> Apparently the device can only handle fixed size packets
> of either 188 or 2*188 byte, thus it breaks with 12288 or 11776.

No.  The device expects 512-byte packets because it uses a bulk 
endpoint.

> The endpoint's wMaxPacketSize might reflect this.

For high-speed devices, a bulk endpoint's wMaxPacketSize must always be 
512.

> I guess a transfer size of e.g. 188*60=11280 would work.
> See the first mail of this thread.

According to Markus, with this particular device nothing but 24064 
works.  The discussion is a little difficult to follow because he 
talked about two different devices without always being clear about 
which was which.

> See also Sergei's comment in
> http://lkml.org/lkml/2011/10/12/183

Alan Stern

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