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SubjectRe: [PATCH]: PCI: GART iommu alignment fixes [v2]


Joerg Roedel wrote:
> 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?

No, I don't have a list. :(

But it seems that the skge driver suffers from this because this code
exists in the driver:

skge->mem = pci_alloc_consistent(hw->pdev, skge->mem_size,
&skge->dma);
if (!skge->mem)
return -ENOMEM;

BUG_ON(skge->dma & 7);

if ((u64)skge->dma >> 32 != ((u64) skge->dma + skge->mem_size)
>> 32) {
printk(KERN_ERR PFX "pci_alloc_consistent region crosses
4G boundary\n");
err = -EINVAL;
goto free_pci_mem;
}


If pci_alloc_consistent did the "right" thing, we should *never* see
that warning message.

In theory, any 32-bit device attempting to request larger than PAGE_SIZE
DMA memory on a system where no memory is available below 4G should show
this problem.

> 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?
>

Yes, this happens if you specify iommu=force on the command line.

P.


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