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: <20090901132824.GN19719@n2100.arm.linux.org.uk>
Date:	Tue, 1 Sep 2009 14:28:24 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Steven Walter <stevenrwalter@...il.com>
Cc:	David Xiao <dxiao@...adcom.com>,
	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 Tue, Aug 25, 2009 at 08:53:29AM -0400, Steven Walter wrote:
> On Thu, Aug 6, 2009 at 6:25 PM, Russell King - ARM
> Linux<linux@....linux.org.uk> wrote:
> [...]
> > As far as userspace DMA coherency, the only way you could do it with
> > current kernel APIs is by using get_user_pages(), creating a scatterlist
> > from those, and then passing it to dma_map_sg().  While the device has
> > ownership of the SG, userspace must _not_ touch the buffer until after
> > DMA has completed.
> [...]
> 
> Would that work on a processor with VIVT caches?  It seems not.  In
> particular, dma_map_page uses page_address to get a virtual address to
> pass to map_single().  map_single() in turn uses this address to
> perform cache maintenance.  Since page_address() returns the kernel
> virtual address, I don't see how any cache-lines for the userspace
> virtual address would get invalidated (for the DMA_FROM_DEVICE case).

You are correct.

> If that's true, then what is the correct way to allow DMA to/from a
> userspace buffer with a VIVT cache?  If not true, what am I missing?

I don't think you read what I said (but I've also forgotten what I did
say).

To put it simply, the kernel does not support DMA direct from userspace
pages.  Solutions which have been proposed in the past only work with a
sub-set of conditions (such as the one above only works with VIPT
caches.)
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