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: <e06498070908250553h5971102x6da7004495abb911@mail.gmail.com>
Date:	Tue, 25 Aug 2009 08:53:29 -0400
From:	Steven Walter <stevenrwalter@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 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).

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?

Thanks
-- 
-Steven Walter <stevenrwalter@...il.com>
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