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

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Date:	Thu, 4 Sep 2008 12:00:35 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	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, 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).

> As I wrote in another mail, you try to bring back the tricks used in
> arch/x86/kernel/pci-dma.c to lib/swiotlb.c. I try to bring back them
> to arch/x86/kernel/pci-swiotlb.c.

As I said, I don't see that as a 'hack'. It is also what the fixme
comment in the swiotlb alloc_coherent function stated, that this
function has to handle the DMA mask of the device.

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

Joerg

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