linux-kernel - Re: [RFC PATCH 0/3] allow fallback to swiotlb on hw iommu init failures

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Message-Id: <20091024120633Z.fujita.tomonori@lab.ntt.co.jp>
Date:	Sat, 24 Oct 2009 12:06:56 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	chrisw@...s-sol.org
Cc:	fujita.tomonori@....ntt.co.jp, dwmw2@...radead.org,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] allow fallback to swiotlb on hw iommu init
	failures

On Fri, 23 Oct 2009 09:39:23 -0700
Chris Wright <chrisw@...s-sol.org> wrote:

> > The concept sounds fine but the third patch doesn't look correct.
> > 
> > Seems that the third patch doesn't take into account enabling both hw
> > iommu and swiotlb (Calgary does and I guess VT-d and AMD need that
> > too).
> 
> VT-d isn't using swiotlb.  Nor is AMD, although I think it will pick up
> no_iommu on passthrough (seems like it would benefit from swiotlb in
> that case).

I think that they need swiotlb for the same reason why Calgray needs
it. IIRC, someone in VT-d camp said so.


> >  Also (iommu_detected && !dma_ops) trick doesn't work for
> > Calgary, IIRC.
> 
> Yes, I think you are right.  I had stared at the calgary code and thought
> it would DTRT due to calgary's use of no_iommu as fallback, but instead
> it will never pick up swiotlb_dma_ops.

Note that Calgary comment 'falling back to no_iommu' is misleading. It
actually falls back to swiotlb or nommu properly.

Calgary doesn't set to dma_ops to calgary_dma_ops so it doesn't need
to pick up swiotlb_dma_ops.


> The calgary shouldn't even need to be manually setting up
> nommu_dma_ops.

Yeah, but it needs because of how the dma startup code works.
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