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: <20090903083600.GA7235@n2100.arm.linux.org.uk>
Date:	Thu, 3 Sep 2009 09:36:00 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Imre Deak <imre.deak@...ia.com>
Cc:	ext Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	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 Wed, Sep 02, 2009 at 06:10:44PM +0300, Imre Deak wrote:
> To my understanding buffers returned by dma_alloc_*, kmalloc, vmalloc
> are ok:

For dma_map_*, the only pages/addresses which are valid to pass are
those returned by get_free_pages() or kmalloc.  Everything else is
not permitted.

Use of vmalloc'd and dma_alloc_* pages with the dma_map_* APIs is invalid
use of the DMA API.  See the notes in the DMA-mapping.txt document
against "dma_map_single".

> For user mappings I think you'd have to do an additional flush for
> the direct mapping, while the user mapping is flushed in dma_map_*.

I will not accept a patch which adds flushing of anything other than
the kernel direct mapping in the dma_map_* functions, so please find
a different approach.
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