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

Chris Pringle
2009-06-18T11:24:52Z

Chris Pringle wrote:
> Chris Pringle wrote:
>>> You could enable CONFIG_NOT_COHERENT_CACHE.
>>>
>> I've just tried this (I had to edit Kconfig in power/platforms to
>> make the build system accept it), and interestingly it's making no
>> difference. I'm using streaming mappings, and are using the
>> pci_map_sg functions to ensure the memory is mapped/flushed
>> correctly. I've also explicitly put in a pci_dma_sync_sg_for_device,
>> however that's also not made any difference. Turning the cpu cache
>> snoop off has the same affect as it did without
>> CONFIG_NOT_COHERENT_CACHE; it gets much worse. Any other ideas?
>>
>> Will back off the low latency patches next, and give 2.6.30 a try -
>> see if that makes any difference.
>>
> Low latency patches made no difference. Tried it with 2.6.30 and it
> now works. There are a couple of commits contributing to the fix,
> including one introduced between 2.6.29-rc8 and 2.6.29 proper in
> powerpc/kernel/head_32.S (couple of commits with the name "Fix Respect
> _PAGE_COHERENT on classic ppc32 SW TLB load machines"). I've tried
> backporting this to 2.6.29-rc8 and it then worked. Backporting to
> 2.6.26 made no difference however, so I suspect there are other things
> fixed which are also contributing.
>
> I'm going to move to 2.6.29/2.6.30 which should resolve our issue.
>
> Thanks to all who have contributed to this thread.
>
The other part of the fix is in asm-powerpc/pgtable32.h. _PAGE_BASE
needs _PAGE_COHERENT in order to work correctly, and in fact there is
now a comment in there to that affect in 2.6.29. Backporting that change
has made it work on 2.6.26. Both this patch, and the fix to head_32.S
are needed for it to work correctly on older kernels.

Chris

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