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:	Wed, 26 Aug 2009 10:22:11 -0700
From:	"David Xiao" <dxiao@...adcom.com>
To:	"Laurent Pinchart" <laurent.pinchart@...asonboard.com>
cc:	"Steven Walter" <stevenrwalter@...il.com>,
	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	"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, 2009-08-25 at 16:17 -0700, Laurent Pinchart wrote:
> On Wednesday 26 August 2009 00:02:48 David Xiao wrote:
> > On Tue, 2009-08-25 at 05:53 -0700, 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).
> > >
> > > 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?
> >
> > page_address() is basically returning page->virtual, which records the
> > virtual/physical mapping for both user/kernel space; and what only
> > matters there is highmem or not.
> 
> I'm not sure to get it. Are you implying that a physical page will then be 
> mapped to the same address in all contexts (kernelspace and userspace 
> processes) ? Is that even possible ? And if not, how could page->virtual store 
> both the initial kernel map and all the userspace mappings ?
> 
Sorry for the confusion, page_address() indeed only returns kernel
virtual address; and in order to support VIVT cache maintenance for the
user space mappings, the dma_map_sg/dma_map_page() functions or even the
struct scatterlist do seem to have to be modified to pass in virtual
address, I think.

David


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