linux-kernel - Re: [PATCH]: PCI: GART iommu alignment fixes [v2]

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Message-Id: <200808071003.47326.jbarnes@virtuousgeek.org>
Date:	Thu, 7 Aug 2008 10:03:43 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>, joro@...tes.org,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH]: PCI: GART iommu alignment fixes [v2]

On Wednesday, August 6, 2008 7:32 am Prarit Bhargava wrote:
> > You can't kmalloc pci_dev or setup some trivial values. You need to
> > use a proper value. The pci code does for us.
>
> Oops -- I meant struct device, not struct pci_dev.
>
> Anwyay, Jesse -- is this true?  I can no longer do something like:
>
>
> static struct device junk_dev = {
>         .bus_id = "junk device",
>         .coherent_dma_mask = 0xffffffff,
>         .dma_mask = &junk_dev.coherent_dma_mask,
> };
>
> And then use that as the device target for dma_alloc_coherent?  AFAIK,
> that has always worked for me.

It gets dangerous since platforms are in control of some pci_dev and dev 
fields, and if they don't get initialized you can get into trouble.

> > Calgary IOMMU has the same code. New AMD IOMMU has the same code too.
>
> Then they don't handle the above problem and are broken when
> dma_get_seg_boundary() returns 0xffffffff and will require patches.
>
> /me hasn't tried out Calgary of AMD IOMMU.

Would be good to find someone to do some testing on one of those platforms...

> >> Maybe I'm missing something -- what implies  size has to be a power of
> >> two?
> >
> > Yes, see iommu_area_alloc().
>
> /me looks and still doesn't see where the size passed into
> gart_map_simple() must be a power of two.  ... and if that was the case,
> shouldn't we be failing all the time?  I mean, I've seen values passed
> into pci_alloc_consistent like 0x3820 -- clearly not a multiple of 2.
>
> iommu_area_alloc() deals with pages, not actual sizes as
> gart_map_simple() does.
>
> If anything, I would make this simple fix:
>
> dma_addr_t map = dma_map_area(dev, paddr, size, dir, size - 1);
>
> should be
>
> dma_addr_t map = dma_map_area(dev, paddr, size, dir, size);
>
> because after my patch we round up the mask argument to get the correct
> alignment to # of pages anyway.

Feel like respinning with a full changelog against my for-linus branch?  Maybe 
you can convince Tomonori-san this time. :)

Jesse
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