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

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Message-Id: <200905221607.34025.arnd@arndb.de>
Date:	Fri, 22 May 2009 16:07:32 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	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, Grant Grundler <grundler@...gle.com>
Subject: Re: [PATCH] asm-generic: add a dma-mapping.h file

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.

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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