linux-kernel - Re: [PATCH] asm-generic: add a dma-mapping.h file

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Date:	Tue, 26 May 2009 13:36:37 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	arnd@...db.de
Cc:	fujita.tomonori@....ntt.co.jp, jgarzik@...ox.com,
	hancockrwd@...il.com, htejun@...il.com, alan@...rguk.ukuu.org.uk,
	flar@...andria.com, schmitz@...phys.uni-duesseldorf.de,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org,
	takata@...ux-m32r.org, geert@...ux-m68k.org,
	linux-m68k@...r.kernel.org, ysato@...rs.sourceforge.jp,
	grundler@...gle.com
Subject: Re: [PATCH] asm-generic: add a dma-mapping.h file

On Fri, 22 May 2009 17:05:53 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> On Friday 22 May 2009, FUJITA Tomonori wrote:
> > I'm not sure. And only mips internally uses CONFIG_DMA_COHERENT.
> > 
> > The reason why many architectures need architecture-specific
> > alloc_coherent() is not about coherent or not.
> > 
> > I like a new helper header file having only generic functions without
> > any ifdef.
> 
> Ok, fair enough. Fixing dma_alloc_coherent to handle
> coherent_dma_mask and debug_dma correctly would also
> make it even bigger, and it was already doing more than
> you'd want from a commonly used inline function.
> It may be useful to put it into kernel/dma-coherent.c
> under an #ifdef, but I'll leave this one alone for now.

Adding #ifdef into kernel/dma-coherent.c is ugly.


> I'll leave dma_alloc_coherent and dma_free_coherent
> as extern declarations then, and leave out the
> simple dma_coherent_dev() and dma_cache_sync() that
> all architectures would need to override.

Right, needs to leave the functions that architectures need to
override.


> dma_get_cache_alignment() is still less generic than
> the other functions, as this is still architecture
> specific. Should I leave that out as well then?

Yes, I think that only adding generic functions is a better
approach. Overriding with #ifdef is really ugly.


> One more idea I had was to rename all the functions in
> this file from dma_* to dma_linear_*. This would mean
> that all architectures using it would still need to
> do something like #define dma_map_sg dma_linear_map_sg
> for each function they want to use but can easily chose
> to provide their own ones for those they need different.
> 
> It might also help architectures that work with dma_ops,
> which could then define their own
> struct dma_ops dma_ops_linear = {
> 	.map_single = dma_linear_map_single,
> 	.map_sg = dma_linear_map_sg,
> 	...
> };

To me, looks like this just makes things complicated needlessly. But
I'm not in a position to ACK or NACK this. it's better to ask
architecture maintainers.


> Would you prefer me to do it this way, or just keep the
> standard function names as in the current patch?
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