linux-kernel - Re: [PATCH 0/3] dma-mapping:remove CONFIG_HAVE_DMA

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Message-ID: <20090705204418.44fc38c1@linux-lm>
Date:	Sun, 5 Jul 2009 20:44:18 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	joerg.roedel@....com, fujita.tomonori@....ntt.co.jp,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 0/3] dma-mapping:remove CONFIG_HAVE_DMA_ATTRS

On Sun, 5 Jul 2009 13:19:36 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> On Saturday 04 July 2009, tom.leiming@...il.com wrote:
> > 2,Disabling CONFIG_HAVE_DMA_ATTRS may lead to a compile failure;
> 
> I'm not sure I understand this point. CONFIG_HAVE_DMA_ATTRS tells
> the common code whether the architecture understands dma attributes.

If a new arch does not define CONFIG_HAVE_DMA_ATTRS but uses
dma-mapping-common.h, it will lead to a compile failure. 

include/asm-generic/dma-mapping-common.h

	#define dma_map_single(d, a, s, r) dma_map_single_attrs(d, a,
	s, r, NULL)
	...

include/linux/dma-mapping.h
	
	#ifndef CONFIG_HAVE_DMA_ATTRS
	struct dma_attrs;

	#define dma_map_single_attrs(dev, cpu_addr, size, dir, attrs) \
        dma_map_single(dev, cpu_addr, size, dir)
	...
	#endif


> If you enable it on all architectures, you will get new compile
> failures on all those that don't understand them, while the current
> code correctly falls back on the standard functions.

You are right, the patch will lead to new compile failure for the
ARCHs, on which disables CONFIG_HAVE_DMA_ATTRS and doesn't use
dma-mapping-common.h. So the patch is not mature.

> 
> I think it makes sense to combine CONFIG_HAVE_DMA_ATTRS with the
> use of dma-mapping-common.h, but the majority of the architectures
> just uses a static mapping, where attributes make no sense.
> 
> You also missed powerpc64, which selects CONFIG_HAVE_DMA_ATTRS
> but does not (yet) use dma-mapping-common.h.

ppc64 doesn't use dma-mapping-common.h, so the patch doesn't consider
it.

-- 
Lei Ming
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