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

From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
To: arnd@arndb.de
Cc: fujita.tomonori@lab.ntt.co.jp, jgarzik@pobox.com,
	hancockrwd@gmail.com, htejun@gmail.com, alan@lxorguk.ukuu.org.uk,
	flar@allandria.com, schmitz@biophys.uni-duesseldorf.de,
	linux-kernel@vger.kernel.org, linux-ide@vger.kernel.org,
	takata@linux-m32r.org, geert@linux-m68k.org,
	linux-m68k@vger.kernel.org, ysato@users.sourceforge.jp,
	grundler@google.com
Subject: Re: [PATCH] asm-generic: add a dma-mapping.h file
Date: Fri, 22 May 2009 23:38:35 +0900	[thread overview]
Message-ID: <20090522233846I.fujita.tomonori@lab.ntt.co.jp> (raw)
In-Reply-To: <200905221607.34025.arnd@arndb.de>

On Fri, 22 May 2009 16:07:32 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> On Friday 22 May 2009, FUJITA Tomonori wrote:
> > I don't think that this works for all architectures because a returned
> > buffer of alloc_pages_node might not be DMA-capable. Needs to use the
> > coherent_dma_mask here. See x86's dma_alloc_coherent and Alpha's
> > pci-noop.c (there might be other examples).
> 
> Right, Grant also made the same comment.
> 
> > I think that having a generic header for simple mapping functions
> > (dma_map_single, dma_map_page, dma_map_sg, etc) would be useful but
> > I'm not sure if we can't have a generic version of
> > dma_alloc_coherent.
> >
> > There are lots of architectures that do 
> > architecture-specific things. For example, you replaced avr32
> > dma-mapping.h except for dma_alloc_coherent, which seems to be can't
> > be generic.
> 
> I agree that it won't be fully generic, because all architectures
> with non-coherent DMA will have to do architecture specific setup
> to get uncached mappings here. I believe that it can be generic
> enough to support all architectures that have a cache-coherent PCI
> bus, hence the #ifdef CONFIG_DMA_COHERENT around this function.

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.

> E.g. cris could use a one-line redirect to
> asm-generic/dma-mapping-linear.h if it did not have the
> special dma_get_cache_alignment function. I still would
> like to find a better way to handle this one than to
> redefine the function.
> 
> > BTW, it looks odd to add dma_debug to dma_map_single, etc but not
> > dma_alloc_coherent. ;)
> 
> hehe, sure. I copied the dma_debug stuff from sh, the comments
> from avr32 and the dma_alloc_coherent implementation from powerpc,
> so I missed this one.
> 
> Thanks for the review.
> 
> 	Arnd <><
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