linux-kernel - Re: [PATCH] sparc: use asm-generic/scatterlist.h

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Message-Id: <20100302230613A.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 2 Mar 2010 23:06:34 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	davem@...emloft.net
Cc:	arnd@...db.de, fujita.tomonori@....ntt.co.jp,
	sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sparc: use asm-generic/scatterlist.h

On Tue, 02 Mar 2010 05:55:34 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> From: Arnd Bergmann <arnd@...db.de>
> Date: Tue, 2 Mar 2010 14:54:11 +0100
> 
> > Broken in the sense that arch/powerpc/Kconfig describes CONFIG_IOMMU_VMERGE:
> > 
> >        Cause IO segments sent to a device for DMA to be merged virtually
> >        by the IOMMU when they happen to have been allocated contiguously.
> >        This doesn't add pressure to the IOMMU allocator. However, some
> >        drivers don't support getting large merged segments coming back
> >        from *_map_sg().
> > 
> >        Most drivers don't have this problem; it is safe to say Y here.
> > 
> > I don't know if this comment still applies to any drivers in the mainline
> > kernel, but it's possible.
> 
> That really has to be out of date these days.

Yeah, I think so.

I added dma_get_max_seg_size() several years ago so that device drives
can tell IOMMU about the maximum segment length that they can
handle. And the default limit (64K) should work for everyone, I think.

I guess that the comment was written when IOMMU was able to merge as
many segments as possible with ignoring the device limitation.
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