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Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is
	get_user_pages() enough to prevent pages from being swapped out ?")
From: Catalin Marinas <catalin.marinas@arm.com>
To: David Xiao <dxiao@broadcom.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
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Organization: ARM Ltd
Date: Tue, 11 Aug 2009 10:31:02 +0100
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On Thu, 2009-08-06 at 22:59 -0700, David Xiao wrote:
> The V7 speculative prefetching will then probably apply to DMA coherency
> issue in general, both kernel and user space DMAs. Could this be
> addressed by inside the dma_unmap_sg/single() calling dma_cache_maint()
> when the direction is DMA_FROM_DEVICE/DMA_BIDIRECTIONAL, to basically
> invalidate the related cache lines in case any filled by prefetching?
> Assuming dma_unmap_sg/single() is called after each DMA operation is
> completed. 

Theoretically, with speculative prefetching on ARMv7 and the FROM_DEVICE
case we need to invalidate the corresponding D-cache lines both before
and after the DMA transfer, i.e. in both dma_map_sg and dma_unmap_sg,
otherwise there is a risk of stale data in the cache.

-- 
Catalin