Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc_coherent

Joerg Roedel
2008-09-10T13:10:56Z

On Wed, Sep 10, 2008 at 10:03:32PM +0900, FUJITA Tomonori wrote:

Ok, I see.

> Hmm, I'm not sure what code you look at. Here's dma_alloc_coherent()
> in tip/x86/iommu:
>
> dma_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_handle,
> gfp_t gfp)
> {
> struct dma_mapping_ops *ops = get_dma_ops(dev);
> void *memory;
>
> gfp &= ~(__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32);
>
> Surely we here clear the flag but...
>
> if (dma_alloc_from_coherent(dev, size, dma_handle, &memory))
> return memory;
>
> if (!dev) {
> dev = &x86_dma_fallback_dev;
> gfp |= GFP_DMA;
> }
>
> we play with it here though (not happens with pci devices),
>
> if (!dev->dma_mask)
> return NULL;
>
> if (!ops->alloc_coherent)
> return NULL;
>
> Then dma_alloc_coherent_gfp_flags() sets it again according to
> device->coherent_dma_mask and gfp before ops->alloc_coherent hook:
>
> return ops->alloc_coherent(dev, size, dma_handle,
> dma_alloc_coherent_gfp_flags(dev, gfp));
>
>
> This code can set up the exact same gfp flag for swiotbl and nommu as
> before.

So its possible that alloc_coherent is called with region specifiers in
the gfp flags. Can't we simply make the gfp hacks depend on
dma_ops->is_phys and avoid further gfp hacks in the hardware iommu
implementations?

Joerg

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