linux-kernel - Re: [PATCH] intel-iommu: Obey coherent_dma_mask for alloc

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Message-Id: <20091106123400J.fujita.tomonori@lab.ntt.co.jp>
Date:	Fri, 6 Nov 2009 12:34:22 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	alex.williamson@...com
Cc:	fujita.tomonori@....ntt.co.jp, dwmw2@...radead.org,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] intel-iommu: Obey coherent_dma_mask for alloc_coherent
 on	passthrough

On Thu, 05 Nov 2009 20:19:52 -0700
Alex Williamson <alex.williamson@...com> wrote:

> On Fri, 2009-11-06 at 11:41 +0900, FUJITA Tomonori wrote:
> > On Wed, 04 Nov 2009 15:59:34 -0700
> > Alex Williamson <alex.williamson@...com> wrote:
> > > @@ -2582,7 +2582,7 @@ static dma_addr_t __intel_map_single(struct device *hwdev, phys_addr_t paddr,
> > >  	BUG_ON(dir == DMA_NONE);
> > >  
> > >  	if (iommu_no_mapping(hwdev))
> > > -		return paddr;
> > > +		return paddr + size > dma_mask ? 0 : paddr;
> > 
> > You can use dma_capable(hwdev, paddr, size) here.
> 
> Good thought, however __intel_map_single() gets called with either the
> dma_mask or the coherent_dma_mask.  dma_capable() only checks dma_mask,
> so would only work for one of the callers.

Oops, you are right.


> > >  	domain = get_valid_domain_for_dev(pdev);
> > >  	if (!domain)
> > > @@ -2767,7 +2767,15 @@ static void *intel_alloc_coherent(struct device *hwdev, size_t size,
> > >  
> > >  	size = PAGE_ALIGN(size);
> > >  	order = get_order(size);
> > > -	flags &= ~(GFP_DMA | GFP_DMA32);
> > > +
> > > +	if (!iommu_no_mapping(hwdev))
> > > +		flags &= ~(GFP_DMA | GFP_DMA32);
> > > +	else if (hwdev->coherent_dma_mask != DMA_BIT_MASK(64)) {
> > > +		if (hwdev->coherent_dma_mask < DMA_BIT_MASK(32))
> > > +			flags |= GFP_DMA;
> > > +		else
> > > +			flags |= GFP_DMA32;
> > > +	}
> > 
> > This is fine for 2.6.32 but we'll cleanly fix this by using
> > swiotlb_dma_ops later, right?
> 
> I'm open to suggestions.  I don't really understand why we dropped
> swiotlb for passthrough mode in 2.6.32 to start with.  It seems like we
> now have a couple corner cases where we have to either hope for the best
> or effectively ignore the request to use passthrough.  Thanks,

I think that the cleanest solution is setting up swiotlb_dma_ops for
passthrough devices (and devices not behind pci, etc). Calgary IOMMU
does the same for years.
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