Re: PCI-DMA: Out of IOMMU space on x86-64 (Athlon64x2), with solution

[Jeff Garzik <jgarzik@pobox.com>]

Andi Kleen wrote:
> On Thursday 02 March 2006 13:16, Jeff Garzik wrote:
> 
> 
>>>Yes I've been thinking about adding a new sleeping interface to the IOMMU
>>>that would block for new space to handle this. If I did that - would
>>>libata be able to use it?
>>
>>No :(  We map inside a spin_lock_irqsave.
> 
> 
> Would it be easily possible to change that or is it difficult?
> 
> Also with the blocking interface there might be possible deadlock issues 
> because it will be essentially similar to allocating memory during IO.
> But I think it's probably safe.

The SCSI layer submits stuff to libata inside spin_lock_irqsave(), and 
from there we DMA-map and send straight to hardware.

So, changing the hot path to permit sleeping would be difficult and add 
needless complexity, IMO.

I would rather pay the penalty of resubmitting if the 
map-inside-spinlock fails, than to slow down the hot path.

	Jeff