linux-kernel - Re: [PATCH] USB: EHCI: fix for leaking isochronous data

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Date:	Thu, 21 Mar 2013 17:06:54 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Soeren Moch <smoch@....de>
cc:	Arnd Bergmann <arnd@...db.de>,
	USB list <linux-usb@...r.kernel.org>,
	Jason Cooper <jason@...edaemon.net>,
	Andrew Lunn <andrew@...n.ch>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	<linux-mm@...ck.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<michael@...rulasolutions.com>
Subject: Re: [PATCH] USB: EHCI: fix for leaking isochronous data

On Thu, 21 Mar 2013, Soeren Moch wrote:

> Now I found out what is going on here:
> 
> In itd_urb_transaction() we allocate 9 iTDs for each URB with 
> number_of_packets == 64 in my case. The iTDs are added to 
> sched->td_list. For a frame-aligned scheduling we need 8 iTDs, the 9th 
> one is released back to the front of the streams free_list in 
> iso_sched_free(). This iTD was cleared after allocation and has a frame 
> number of 0 now. So for each allocation when now_frame == 0 we allocate 
> from the dma_pool, not from the free_list.

Okay, that is a problem.  But it shouldn't be such a big problem,
because now_frame should not be equal to 0 very often.

>  The attached patch 
> invalidates the frame number in each iTD before it is sent to the 
> scheduler. This fixes the problem without the need to iterate over a iTD 
> list.

The patch looks okay.  However I would like to understand why the 0 
frame value messes things up so much.

Alan Stern

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