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:	Tue, 1 Sep 2009 17:53:48 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Steven Walter <stevenrwalter@...il.com>,
	David Xiao <dxiao@...adcom.com>,
	Ben Dooks <ben-linux@...ff.org>, Robin Holt <holt@....com>,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	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 Tue, 1 Sep 2009, Russell King - ARM Linux wrote:
> On Tue, Sep 01, 2009 at 03:43:48PM +0200, Laurent Pinchart wrote:
> > I might be missing something obvious, but I fail to see how VIVT caches
> > could work at all with multiple mappings. If a kernel-allocated buffer
> > is DMA'ed to, we certainly want to invalidate all cache lines that store
> > buffer data. As the cache doesn't care about physical addresses we thus
> > need to invalidate all virtual mappings for the buffer. If the buffer is
> > mmap'ed in userspace I don't see how that would be done.
> 
> You need to ask MM gurus about that.  I don't touch the Linux MM very
> often so tend to keep forgetting how it works.  However, it does work
> for shared mappings of files on CPUs with VIVT caches.

I believe arch/arm/mm/flush.c __flush_dcache_aliases() is what does it.

Hugh
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