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

From: Chris Pringle
Date: Wed Jun 17 2009 - 09:18:54 EST

Chris Pringle wrote:

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.



Chris Pringle
Software Engineer

Miranda Technologies Ltd.
Hithercroft Road
Oxfordshire OX10 9DG

Tel. +44 1491 820206
Fax. +44 1491 820001


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