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SubjectRe: [PATCH] pci: print out DMA mask info
On Fri, Oct 10, 2008 at 11:40:56AM +0900, FUJITA Tomonori wrote:
> IIRC, except for one SGI architecture, coherent_dma_mask is
> meaningless, dma_mask is always equal to coherent_dma_mask.

Not correct. Several PCI-X devices can only do 32-bit DMA to control
data but can do 64-bit DMA for payload data. I don't have the list off
the top of my head but that is the origin of coherent DMA mask.

> Lots of
> IOMMU implementations ignore coherent_dma_mask and use dma_mask for
> alloc_coherent().

That sounds like a bug. And I don't think it's "lots".

> Some drivers don't set up coherent_dma_mask.

They probably should. Current default is 32-bits for PCI devices.

> Theoretically, we need to fix this but it doesn't cause any
> problem. That's why nobody cares about it, I guess.

Agreed - for the "supported" configurations, it works.


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