linux-kernel - Re: [RFC][PATCH] add dma_reserve_coherent_memory()/dma_free_reserved

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Date:	Thu, 26 Aug 2010 18:54:40 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	u.kleine-koenig@...gutronix.de, mitov@...p.bas.bg,
	linux-sh@...r.kernel.org, gregkh@...e.de,
	linux-kernel@...r.kernel.org, jkrzyszt@....icnet.pl,
	philippe.retornaz@...l.ch, akpm@...ux-foundation.org,
	g.liakhovetski@....de, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org
Subject: Re: [RFC][PATCH] add
	dma_reserve_coherent_memory()/dma_free_reserved_memory() API

On Thu, Aug 26, 2010 at 07:00:24PM +0900, FUJITA Tomonori wrote:
> On Thu, 26 Aug 2010 11:53:11 +0200
> Uwe Kleine-König <u.kleine-koenig@...gutronix.de> wrote:
> 
> > > > We have currently a number of boards broken in the mainline. They must be 
> > > > fixed for 2.6.36. I don't think the mentioned API will do this for us. So, 
> > > > as I suggested earlier, we need either this or my patch series
> > > > 
> > > > http://thread.gmane.org/gmane.linux.ports.sh.devel/8595
> > > > 
> > > > for 2.6.36.
> > > 
> > > Why can't you revert a commit that causes the regression?
> > > 
> > > The related DMA API wasn't changed in 2.6.36-rc1. The DMA API is not
> > > responsible for the regression. And the patchset even exnteds the
> > > definition of the DMA API (dma_declare_coherent_memory). Such change
> > > shouldn't applied after rc1. I think that DMA-API.txt says that
> > > dma_declare_coherent_memory() handles coherent memory for a particular
> > > device. It's not for the API that reserves coherent memory that can be
> > > used for any device for a single device.
> > The patch that made the problem obvious for ARM is
> > 309caa9cc6ff39d261264ec4ff10e29489afc8f8 aka v2.6.36-rc1~591^2~2^4~12.
> > So this went in before v2.6.36-rc1.  One of the "architectures which
> > similar restrictions" is x86 BTW.
> > 
> > And no, we won't revert 309caa9cc6ff39d261264ec4ff10e29489afc8f8 as it
> > addresses a hardware restriction.
> 
> How these drivers were able to work without hitting the hardware restriction?

Well, OMAP processors have experienced lock-ups due to multiple mappings of
memory, so the restriction in the architecture manual is for real.

But more the issue is that the behaviour you get from a region is _totally_
unpredictable (as the arch manual says).  With the VIPT caches, they can
be searched irrespective of whether the page tabkes indicate that it's
supposed to be cached or not - which means you can still hit cache lines
for an ioremap'd region.

And if you do, how do you know that the cached data is still valid - what
if it's some critical data that results in corruption - how do you know
whether that's happened or not?  It might not even cause a kernel
exception.

We have to adhere to the restrictions placed upon us by the architecture
at hand, and if that means device drivers break, so be it - at least we
get to know what needs to be fixed for these restrictions.
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