Re: [openib-general] Suggestions for how to remove bus_to_virt()

[Muli Ben-Yehuda <muli@il.ibm.com>]

On Wed, Jul 12, 2006 at 05:40:13PM -0700, David Miller wrote:
> From: Roland Dreier <rdreier@cisco.com>
> Date: Wed, 12 Jul 2006 17:11:26 -0700
> 
> > A cleaner solution would be to make the dma_ API really use the device
> > it's passed anyway, and allow drivers to override the standard PCI
> > stuff nicely.  But that would be major surgery, I guess.
> 
> Clean but expensive, you should not force the rest of the kernel
> to eat the cost of something you want to do when it's totally
> unnecessary for most other users.
> 
> For example, x86 never needs to do anything other than a direct
> virt_to_phys translation to produce a DMA address, no matter what
> bus the device is on.  It's a single simple integer adjustment
> that can be done inline in about 2 or 3 instructions at most.

It's possible that even x86 will support multiple IOMMUs in the future
- for example, the Calgary IOMMU support we recently added to x86-64
could be modified to work on plain x86 as well.

I like the idea of a per-device DMA-API implementation, but only if it
can be done in a way that is zero cost to the majority of the users of
the API. We already have dynamic dma_ops on x86-64 to support nommu,
swiotlb, gart and Calgary cleanly, extending it to use a per-device
dma-ops isn't too difficult.

Cheers,
Muli