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: <412A05BA40734D4887DBC67661F433080D73232D@EXMAIL.ad.emulex.com>
Date:	Thu, 6 Aug 2009 12:16:35 -0700
From:	Chetan.Loke@...lex.Com
To:	<dxiao@...adcom.com>, <laurent.pinchart@...asonboard.com>
CC:	<ben-linux@...ff.org>, <hugh.dickins@...cali.co.uk>,
	<holt@....com>, <linux-kernel@...r.kernel.org>,
	<linux-media@...r.kernel.org>,
	<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 ?")

> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-
> owner@...r.kernel.org] On Behalf Of David Xiao
> Sent: Thursday, August 06, 2009 2:46 PM
> To: Laurent Pinchart
> Cc: Ben Dooks; Hugh Dickins; Robin Holt; linux-kernel@...r.kernel.org;
> v4l2_linux; 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, 2009-08-06 at 06:06 -0700, Laurent Pinchart wrote:
> > Hi Ben,
> >
> > On Thursday 06 August 2009 13:46:19 Ben Dooks wrote:
> > > On Thu, Aug 06, 2009 at 12:08:21PM +0200, Laurent Pinchart wrote:
> > [snip]
> > > >
> > > > The second problem is to ensure cache coherency. As the userspace
> > > > application will read data from the video buffers, those buffers
> will end
> > > > up being cached in the processor's data cache. The driver does need
> to
> > > > invalidate the cache before starting the DMA operation (userspace
> could
> > > > in theory write to the buffers, but the data will be overwritten by
> DMA
> > > > anyway, so there's no need to clean the cache).
> > >
> > > You'll need to clean the write buffers, otherwise the CPU may have
> data
> > > queued that it has yet to write back to memory.
> >
> > Good points, thanks.
> 
>    I thought this should have been taken care of by the CPU specific
> dma_inv_range routine. However, In arch/arm/mm/cache-v7.c,
> v7_dma_inv_range does not drain the write buffer; and the
> v6_dma_inv_range does that in the end of all the cache maintenance
> operaitons.
>    So this is probably something Russel can clarify.
> 

Something non-related. I haven't used this specific api but ARM1156 has an issue. If you use the clean-cache-block mcr feature then it might result in memory-corruption. So be careful. I'm not sure which of these(ARM1156T2-S or ARM1156T2F-S) variants has that errata. 


Chetan
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