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

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Date:	Thu, 21 Mar 2013 17:12:01 -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, 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.

Very good; tomorrow I will send your patch in.

Alan Stern

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