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

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Message-Id: <200906162002.12376.arnd@arndb.de>
Date:	Tue, 16 Jun 2009 20:02:11 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Scott Wood <scottwood@...escale.com>
Cc:	linuxppc-dev@...ts.ozlabs.org,
	Chris Pringle <chris.pringle@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: PowerPC PCI DMA issues (prefetch/coherency?)

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).

Actually there seems to be a bug in here: Since dma_alloc_noncoherent
gives you a coherent mapping (or NULL) on noncoherent machines,
dma_cache_sync() is redundant and should not actually flush the
cache, or we should change dma_alloc_noncoherent to do a simple
alloc_pages on CONFIG_NON_COHERENT_CACHE and leave dma_cache_sync()
as it is.

	Arnd <><
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