Messages in this thread | | | Date | Thu, 24 Jul 2008 06:09:31 -0400 | From | Prarit Bhargava <> | Subject | Re: [PATCH]: PCI: GART iommu alignment fixes [v2] |
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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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