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

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Message-ID: <4A37DB0F.7060801@freescale.com>
Date:	Tue, 16 Jun 2009 12:49:03 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Arnd Bergmann <arnd@...db.de>
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?)

Arnd Bergmann wrote:
> On Tuesday 16 June 2009, Scott Wood wrote:
>> Chris Pringle wrote:
>>> Ah right - that would explain what we're seeing then... Doh. Thought I 
>>> might have been onto something then. Is there any way to force a cache 
>>> flush? That'd at least prove it was a caching issue if it resolved the 
>>> problem.
>> You could enable CONFIG_NOT_COHERENT_CACHE.
> 
> If the whole system is noncoherent, that is the right solution.

I meant it more as a test than a permanent solution...

> 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?

-Scott
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