linux-kernel - Re: [PATCH 1/3] x86: remove the NULL device hack in dma-mapping.h

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Message-Id: <20080904215030Y.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 4 Sep 2008 21:50:19 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	joerg.roedel@....com
Cc:	fujita.tomonori@....ntt.co.jp, joro@...tes.org,
	tony.luck@...el.com, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org, mingo@...hat.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 1/3] x86: remove the NULL device hack in dma-mapping.h

On Thu, 4 Sep 2008 12:00:35 +0200
Joerg Roedel <joerg.roedel@....com> wrote:

> On Thu, Sep 04, 2008 at 01:11:46PM +0900, FUJITA Tomonori wrote:
> > On Wed, 3 Sep 2008 22:01:14 +0200
> > Joerg Roedel <joro@...tes.org> wrote:
> > 
> > > On Thu, Sep 04, 2008 at 03:04:23AM +0900, FUJITA Tomonori wrote:
> > > > dma_alloc_coherent in dma-mapping.h has a hack to use
> > > > x86_dma_fallback_dev if a pointer to a device is NULL. Some of IOMMUs
> > > > don't need such hack. The hack also makes it difficult for IOMMUs to
> > > > make a proper decision because the hack hides the information.
> > > 
> > > I don't think its the right way to work around shortcomings of the
> > > generic code in the architecture specific implementations. Especially
> > > when the generic code can be easily fixed like in this case.
> > 
> > Well, the generic code should not have such work around.
> 
> I don't see that as a workaround. It is the best what we can do to
> handle device dma_masks with the current Linux page allocator (if we
> don't have hardware dma translation).

It's a hack. What swiotlb should do is allocating from only swiotlb's
memory area (I agree with Andi about this). dma_mask is irrelevant for
it. The current swiotlb_alloc_coherent is a bit wrong though I don't
like to bother IA64 people.


> > > > +static void *x86_swiotlb_alloc_coherent(struct device *dev, size_t size,
> > > > +					dma_addr_t *dma_handle, gfp_t gfp)
> > > > +{
> > > > +	if (!dev) {
> > > > +		dev = &x86_dma_fallback_dev;
> > > > +		gfp |= GFP_DMA;
> > > > +	}
> > > 
> > > This really should be checked in the generic x86 dma_alloc_coherent
> > > function.
> > 
> > I don't think so. Any motherboards with the recent IOMMUs support ISA?
> 
> Not that I am aware of. But as we both know there are people who do
> corner case tests with the dma-api functions like passing their own
> created devices or even NULL to it an look what happens :-)

That's the wrong way to use DMA API. We don't need to care about
it. The only reason we handle it now is for the ancient code.


> We have to handle this case in _every_ IOMMU implemention. So the
> generic function is the right place for this check, imho.

As I said, I don't think that every IOMMU need to handle it. Well,
it's a minor issue. I don't care much so handling it in a generic code
is fine. But as I pointed out in another mail, the fallback device has
DMA_32BIT_MASK so hiding it in dma-mapping.h makes it difficult to
make pci-dma.c work as before.
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