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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Date:	Fri, 7 Aug 2009 11:23:39 +0100
From:	Jamie Lokier <jamie@...reable.org>
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 ?")

David Xiao wrote:
> > However, that won't work with ARMv7's speculative prefetching.  I'm
> > afraid with such things, DMA direct into userspace mappings becomes a
> > _lot_ harder, and lets face it, lots of Linux drivers just aren't going
> > to bother supporting this - we can't currently get agreement to have an
> > API to map DMA coherent pages into userspace!
> 
> 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. 

If it's possible, surely its essential because of O_DIRECT file and
block I/O?

-- Jamie
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