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SubjectRe: [PATCH]: PCI: GART iommu alignment fixes [v2]
On Wed, Jul 23, 2008 at 07:47:03PM -0400, Prarit Bhargava wrote:
>
> >>Interesting. Have you experienced any problems because of that
> >>misbehavior in the GART code? AMD IOMMU currently also violates this
> >>requirement. I will send a patch to fix that there too.
> >>
> >
> >
>
> Joerg, yes I can see misbehavior caused by this code. O/w I wouldn't
> be spending my time fixing it :) :)
>
> See below ....
>
> >IIRC, only PARISC and POWER IOMMUs follow the above rule. So I also
> >wondered what problem he hit.
> >
>
> I wonder if IBM's Calgary IOMMU needs this fix? ... I've added Ed
> Pollard to find out.
>
> On big memory footprint (16G or above) systems it is possible that the
> e820 map reserves most of the lower 4G of memory for system use*. So
> it's possible that the 4G region is almost completely reserved at boot
> time and so the kernel starts using the IOMMU for DMA (see
> dma_alloc_coherent()). The addresses returned are not properly aligned,
> and this causes serious problems for some drivers that require a
> physical aligned address for the device.

Do you have a list of driver which require this? I would like to
reproduce this issue. Does it also happen when you start the kernel with
iommu=force (GART should then be used for all DMA remapping) too?

Joerg


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