Re: PowerPC PCI DMA issues (prefetch/coherency?)

Chris Pringle
2009-06-16T16:57:46Z

Scott Wood wrote:
>
> Check asm/cputable.h for CPU_FTR_NEED_COHERENT. Make sure that
> CONFIG_8260 is one of the #ifdefs that turns that on. It looks like
> that was in place by 2.6.26 in arch/powerpc. I'm not sure what to
> look for in arch/ppc.
I've just checked that and it's definitely switched on in CPU_FTR_COMMON
(CONFIG_8260 is also being used).
>
>>> Also make sure that you park the bus on PCI and raise its arbitration
>>> priority, as done at the end of fixup_pci in
>>> arch/powerpc/boot/cuboot-pq2.c.
>>>
>> Since this is a reasonably recent kernel,
>
> Not really, there was a fair amount of 82xx work in the mid-2.6.20s.
> The addition of CPU_FTR_NEED_COHERENT to 82xx was somewhere in that time.
>
> Can you try 2.6.30?
I'll give it a try, but that won't be a quick thing to do - will
hopefully manage to get that done tomorrow if it patches without too
many issues. I should point out that we've got the low latency patches
on this kernel too; I guess it'd be worth trying it without them before
I move kernels.
>
>> I'd guess that both of these things are correct. I've had a quick
>> look in that file and there is code in there raising arbitartion
>> priority and parking the bus.
>
> Just because the code is there doesn't mean you're using it -- are you
> using cuImage? Are you using arch/ppc or arch/powerpc?
>
> Typically this would be done by firmware; it's only in cuboot because
> u-boot wasn't doing it.
Just checked this is being called and it is. We're using arch/powerpc.
>
>
>> Interestingly, I've just turned off cache snooping and the problem
>> has got much worse. This has surprised me as I thought that part of
>> the job done by pci_map_sg was to flush the CPU cache
>
> It only flushes the cache on hardware that doesn't do coherent DMA.
>
Ah right - that would explain what we're seeing then... Doh. Thought I
might have been onto something then. Is there any way to force a cache
flush? That'd at least prove it was a caching issue if it resolved the
problem.

Thanks,
Chris

--

______________________________
Chris Pringle
Software Engineer

Miranda Technologies Ltd.
Hithercroft Road
Wallingford
Oxfordshire OX10 9DG
UK

Tel. +44 1491 820206
Fax. +44 1491 820001
www.miranda.com

____________________________

Miranda Technologies Limited
Registered in England and Wales CN 02017053
Registered Office: James House, Mere Park, Dedmere Road, Marlow, Bucks, SL7 1FJ