Re: [RFC] block layer support for DMA IOMMU bypass mode II

[Andi Kleen]

On Thu, Jul 03, 2003 at 09:26:29PM +0100, Alan Cox wrote:
> On Iau, 2003-07-03 at 00:56, Andi Kleen wrote:
> > > 1.	We allocate pages in reverse order so most merges cant occur
> > 
> > I added an printk and I get quite a lot of merges during bootup
> > with normal IDE.
> > 
> > (sometimes 12+ segments) 
> 
> Thats merging adjacent blocks with non adjacent page targets using the
> IOMMU right - I was doing mergign without an IOMMU which is a little

Yep. 

> different and turns out to be a waste of cpu

Understandable. Especially when memory fragments after some uptime.

But of course it doesn't help much in practice because all the interesting
block devices support DAC anyways and the IOMMU is disabled for that.

Also it's likely cheaper just submit more segments than to have the IOMMU
overhead
(at least for sane devices, if not it may be worth to artificially limit the
dma mask of the device to force IOMMU on IA64 and x86-64) 

-Andi