linux-kernel - Re: PowerPC PCI DMA issues (prefetch/coherency?)

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Message-Id: <20090617093654B.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 17 Jun 2009 09:37:14 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	benh@...nel.crashing.org
Cc:	arnd@...db.de, scottwood@...escale.com, chris.pringle@...el.com,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: PowerPC PCI DMA issues (prefetch/coherency?)

On Wed, 17 Jun 2009 10:18:45 +1000
Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> On Tue, 2009-06-16 at 20:02 +0200, Arnd Bergmann wrote:
> > On Tuesday 16 June 2009, Scott Wood wrote:
> > > > If the
> > > > device is the only one, you can also use dma_alloc_noncoherent() and
> > > > flush explicitly with dma_cache_sync().
> > > 
> > > I don't see how that would help -- aren't those also controlled by 
> > > CONFIG_NOT_COHERENT_CACHE?
> > 
> > Ah, yes you are right. PowerPC implements dma_alloc_noncoherent as
> > dma_alloc_coherent, so dma_cache_sync() is actually a NOP (or should be).
> 
> But we still need to sync the result of dma_map_* when used multiple
> times for a single mapping.

We have dma_sync_{single|sg}_for_{cpu|device} API for the above
purpose.

dma_cache_sync is supposed to be used only with the buffers that
dma_alloc_noncoherent() returns. On architecutures that maps
dma_alloc_noncoherent to dma_alloc_coherent, dma_cache_sync() is
supposed to be NOP.
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