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

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Date:	Thu, 21 Mar 2013 22:45:16 +0100
From:	Soeren Moch <smoch@....de>
To:	Alan Stern <stern@...land.harvard.edu>
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 03/21/13 22:12, Alan Stern wrote:
> On Thu, 21 Mar 2013, Alan Stern wrote:
>
>> 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.
>
> Oh, wait, now I get it.  We never reach a steady state, because the
> free list never shrinks, but occasionally it does increase when
> now_frame is equal to 0.  Even though that doesn't happen very often,
> the effects add up.

Correct. Approximately twice a second we allocate 9 iTDs from the 
dma_pool in my case (the frame numbers run from 0 to 511 - not up to 
2047, which I thought they should). But since we allocate iTDs every 8 
frames in my case (for each active stream), the pool allocations occur 
only when we hit the allocation sequence frame 0,8,16,... (not one of 
the other 7 opportunities frame 1,9,17,... - frame 7,15,23,...)

> Very good; tomorrow I will send your patch in.
>
> Alan Stern
>
Thank you. And also thanks to all who helped to track down this issue.

  Soeren
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