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

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Message-Id: <20091028155251C.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 28 Oct 2009 15:53:53 +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 23:57:12 -0700
Chris Wright <chrisw@...s-sol.org> wrote:

> > 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.
> 
> It does need swiotlb_dma_ops even when calgary init succeeds for the devices
> that aren't behind Calgary to deal w/ the case of those devices having
> dma mask smaller than physical memory (i.e those that don't get device
> specific dma_ops set to calgary_dma_ops).

I know since I wrote that code.


> > > 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.
> 
> Be much better to have the core handle all of this. Basically register
> ops and then put the top one on the stack to actual use.  So the
> fallback would be automatic, just pick the top of the stack and go.
> Were you thinking of something along those lines?

I don't know what 'register ops and then put the top one on the stack'
means but it sounds overdoing.

I think that we can handle this issue more simply. I've just posted
the patchset.
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