linux-kernel - Re: [PATCH 0/3] fix GART to respect device's dma

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Message-Id: <20080923130156O.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 23 Sep 2008 13:02:04 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	andi@...stfloor.org
Cc:	fujita.tomonori@....ntt.co.jp, mingo@...e.hu, joerg.roedel@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] fix GART to respect device's dma_mask about
 virtual mappings

On Mon, 22 Sep 2008 22:35:18 +0200
Andi Kleen <andi@...stfloor.org> wrote:

> On Tue, Sep 23, 2008 at 04:12:33AM +0900, FUJITA Tomonori wrote:
> > Sure, pci-nommu's map_sg/map_single doesn't handle it. But we handle
> > this issue somewhere else (like b44 keeps own DMA buffer)?
> 
> Yes, b44 handles it on its own. It has to for 32bit which always
> has a nop map_sg/single. Also some other subsystems like the block layer
> do it implicitely.

Yeah, 'somewhere else' includes the block layer bounce.


> > 1. not all map_sg/map_single (e.g. pci-nommu) can't handle it.
> 
> and especially i386
> 
> > 2. we already have workarounds for it somewhere else so
> > map_sg/map_single don't need to handle it.
> > 3. I changed GART map_sg/map_signle to handle it. I thought if it
> > can handle it, for example, b44 doesn't go the workaround path. It
> > would be a good thing.
> > 4. But GART cannot remap to addresses < 4GB reliably, so my above
> > argument doesn't always work.
> > 5. Then my patch doesn't break anything but it's almost meaningless.
> 
> Correct.

Thanks for the clarification.
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