[PATCH] [RFC] EHCI: add to memory barrier to updating hw_next
tom.leiming at gmail.com
Thu Jul 18 06:07:19 EDT 2013
On Thu, Jul 18, 2013 at 9:30 AM, Gioh Kim <gioh.kim at lge.com> wrote:
> Thanks for your reply.
>> -----Original Message-----
>> From: Ming Lei [mailto:tom.leiming at gmail.com]
>> Sent: Wednesday, July 17, 2013 5:52 PM
>> To: 김기오
>> Cc: Alan Stern; linux-usb at vger.kernel.org; linux-kernel at vger.kernel.org;
>> Mark Salter; namhyung.kim at lge.com; Minchan Kim; Chanho Min; Jong-Sung Kim;
>> Subject: Re: [PATCH] [RFC] EHCI: add to memory barrier to updating hw_next
>> Cc: ARM list
>> On Wed, Jul 17, 2013 at 1:03 PM, 김기오 <gioh.kim at lge.com> wrote:
>> > Hi,
>> > I have a missing urb completion problem on ARMv7 based platform.
>> > I thought the above problem was caused by coherent memory between the
>> > EHCI device and CPU so I tryied to allocates device type memory for
>> > EHCI via dma_declare_coherent_memory at machine initialization step so
>> > that EHCI always allocates from those device type memory.
>> > It seems to solve the issue because I didn't see any problem.
>> > But I am not sure it is acceptable solution. So I applied the patch
>> > https://lkml.org/lkml/2011/8/31/344.
>> > But it could not solve the problem so that I added another wmb() as my
>> > patch, and now my platform works fine.
>> I remember the previous problem reported on Pandaboard firstly was fixed
>> by Will's commit 11ed0ba1(ARM: 7161/1: errata: no automatic store buffer
>> drain), so could you try to enable PL310_ERRATA_769419 and see if your
>> problem can be fixed?
> I also know the errata but it didn't work for my platform.
>> > I am not sure what's the exact problem and what wmb I added could
>> > solve but I just think the problem is related to store buffer flush of
>> Yes, per the above commit, but it might be one hardware problem.
>> > Anyway, important thing is that it fixed my problem so I expect you
>> > expert guys could find what I am missing and a right solution.
>> > IMHO, the patch might miss updating hw_next pointer.
>> > Am I correct?
>> > I understand the wmb() is just memory barrier, not write-buffer flush.
>> > But it is true that wmb() can flush write buffer in ARM.
>> > Anyhow I think that memory type, "normal memory, non-cacheable", may
>> > have a problem for some devices that needs device type memory.
>> Currently on ARMv7 DMA coherent buffer has to be bufferable, in theory we
>> might need one API to flush CPU write buffer, as described in
>> Also, on some platforms your driver may need to flush CPU write
>> buffers in much the same way as it needs to flush write buffers
>> found in PCI bridges (such as by reading a register's value
>> after writing it).
>> But actually on hardware without the problem(such as A15), I don't see any
>> effect without flushing store buffer explicitly.
> The problem of my platform is occurring when it has very heavy traffic such as
> HD video streaming service or very many small images display.
Could you describe your test case in a bit detail? Which USB
may trigger your URB completion miss problem? Under what kind of video
streaming service(USB video or only kind of heavy load)?
> I guess that HC could have a use-after-free problem like following situation.
> 1. A qtd which is not at the queue head should be removed in qh_completions().
> 2. The last->hw_next become be pointing at the next qtd but the hw_next value is delayed in write-buffer.
> 3. The qtd is removed in the list.
> 4. The qtd is freed into DMA pool and re-allocated for another urb.
> 5. HC try to process last->hw_next and it is pointing re-allocated qtd.
> What do you think about it? Is it possible?
I understand it might not be possible because: when 'stopped' is set, that
said the HC might not advance the queue. But I don't understand why
'last->hw_next' is patched here under 'stopped' situation.
Even the 'stopped' case may be seldom triggered, do you know under
which condition the stopped is triggered in your problem?(stall, short read
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