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: <20090901141812.GT19719@n2100.arm.linux.org.uk>
Date:	Tue, 1 Sep 2009 15:18:12 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Steven Walter <stevenrwalter@...il.com>,
	David Xiao <dxiao@...adcom.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 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.
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