linux-kernel - Re: scatterlist.c or dma_map

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Message-Id: <20080929021041R.fujita.tomonori@lab.ntt.co.jp>
Date:	Mon, 29 Sep 2008 02:11:38 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	leon.woestenberg@...il.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: scatterlist.c or dma_map_sg() should coalesce?

On Sun, 28 Sep 2008 17:47:59 +0200
"Leon Woestenberg" <leon.woestenberg@...il.com> wrote:

> Hello,
> 
> in 2.6.27+, if I want to create a scatterlist of a large buffer in
> virtual memory in a device driver, can I expect the scatterlist and/or
> mapping API to provide me with a minimal list, each entry describing a
> maximum bus-contiguous piece of memory? (i.e. can I assume coalescing
> is done for me?).

No, it's not the job of dma_map_sg. Before dma_map_sg, physical
continuous page frames in a scatter list are supposed be coalesced.
For example, the block layer creates a scatter list in which physical
continuous page frames are coalesced for SCSI drivers.

> In 2.6.24, dma_map_sg() *may* coalesce, but doesn't do so for my
> platform, so for optimal DMA transfers the device driver needs to
> coalesce the list entries.

With some of IOMMUs, dma_map_sg might coalesce virtual and physical
continuous page frames. If 2.6.24 doesn't do for you, 2.6.27 doesn't
too.
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