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

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Message-ID: <4A3A23FA.1000407@oxtel.com>
Date:	Thu, 18 Jun 2009 12:24:42 +0100
From:	Chris Pringle <chris.pringle@...el.com>
To:	Scott Wood <scottwood@...escale.com>
CC:	"linuxppc-dev@...abs.org list" <linuxppc-dev@...ts.ozlabs.org>,
	linux-kernel@...r.kernel.org
Subject: Re: PowerPC PCI DMA issues (prefetch/coherency?)

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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