linux-kernel - Re: [PATCH 8/8] remove map_single and unmap_single in struct dma_mapping

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Message-Id: <20090106192022D.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 6 Jan 2009 19:22:08 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	joro@...tes.org
Cc:	fujita.tomonori@....ntt.co.jp, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] remove map_single and unmap_single in struct
 dma_mapping_ops

On Mon, 5 Jan 2009 19:00:38 +0100
Joerg Roedel <joro@...tes.org> wrote:

> Is it the right way to implement map_single in terms of map_page? Doing
> this you optimize for the map_page case. But a grep in drivers/ shows:
> 
> linux/drivers $ grep -r _map_page *|wc -l
> 126
> linux/drivers $ grep -r _map_single *|wc -l
> 613

The comparison is irrelevant since dma_map_page and dma_map_single
have different purposes.

If passing virtual memory address to an IOMMU is enough (and
convenient), then drivers use dma_map_single.

For some purposes, drivers need to pass a page frame and use
dma_map_page (or dma_map_sg).

We could have two hooks in dma_map_ops struct for dma_map_single and
dma_map_page. Say, we have map_single and map_page hooks. But the
map_page hook can be used to support both dma_map_single and
dma_map_page. Note that the map_single hook can't do that since it use
a virtual address as an argument. That's why I have only the map_page
hook in dma_map_ops struct.

As X86 does now, we could have map_single hook that use a physical
address to handle both dma_map_single and dma_map_page. However, it's
confusing since it means that the arguments of dma_map_single and its
hook (map_single) is inconsistent.

> There are a lot more users of map_single than of map_page. I think its
> better to optimize for the map_single case and implement map_page in
> terms of map_single.

As I wrote above, it doesn't make sense.
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