Re: 32-bit dma allocations on 64-bit platforms

Andi Kleen
2004-06-24T10:33:20Z

On Thu, Jun 24, 2004 at 08:18:06AM +0200, Arjan van de Ven wrote:
> On Wed, 2004-06-23 at 23:46, Andi Kleen wrote:
>
> > The VM should be able to handle this, but it may still require
> > some tuning. It would need some generic changes, but not too bad.
> > Still would need a decision on how big GFP_BIGDMA should be.
> > I suspect 4GB would be too big again.
>
> What is the problem again, can't the driver us the dynamic pci mapping
> API which does allow more memory to be mapped even on crippled machines
> without iommu ?

In theory one could fix pci_alloc_consistent from the swiotlb pool yes,
the problem is just that this pool is completely preallocated. If
enough memory is needed that would be quite nasty, because you suddenly
lose 1 or 2GB RAM.

> And isn't this a problem that will vanish since PCI Express and PCI X
> both *require* support for 64 bit addressing, so all higher speed cards
> are going to be ok in principle ?

There are EM64T systems with AGP only and not all PCI-Express cards
seem to follow this. PCI-Express unfortunately discouraged the AGP aperture too,
so not even that can be used on those Intel systems.

-Andi