linux-kernel - Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")

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Message-Id: <1249983062.27150.20.camel@pc1117.cambridge.arm.com>
Date:	Tue, 11 Aug 2009 10:31:02 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	David Xiao <dxiao@...adcom.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Robin Holt <holt@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	v4l2_linux <linux-media@...r.kernel.org>,
	"linux-arm-kernel@...ts.arm.linux.org.uk" 
	<linux-arm-kernel@...ts.arm.linux.org.uk>
Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is
	get_user_pages() enough to prevent pages from being swapped out ?")

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

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