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

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Date:	Wed, 17 Jun 2009 10:18:45 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Scott Wood <scottwood@...escale.com>,
	Chris Pringle <chris.pringle@...el.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: PowerPC PCI DMA issues (prefetch/coherency?)

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.

Cheers,
Ben.

> 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 <><
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@...ts.ozlabs.org
> https://lists.ozlabs.org/listinfo/linuxppc-dev

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