Re: [PATCH 1/3] iscsi iser: remove DMA restrictions

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Thu, 2008-02-14 at 11:56 -0600, Mike Christie wrote:
> > You really don't want to do this.  That signals to the block layer that
> > we have an iommu, although it's practically the same thing as a 64 bit
> > DMA mask ... but I'd just leave it to the DMA mask to set this up
> > correctly.  Anything else is asking for a subtle bug to turn up years
> > from now when something causes the mask and the limit to be mismatched.
> > 
> 
> I thought BLK_BOUNCE_ANY just meant "don't bounce anything" (that was 
> from the blkdev.h comments). 

It does ... that's why it's used in the IOMMU case ... and why it's
practically the same as a 64 bit mask.

> We used it for iscsi_tcp because the network layer can take any type
> of page and will do the right thing for the hardware it eventually
> gets sent to.

Right, to you it means never bounce because net wants to do it instead.
However, I don't think that's the case for iSER, is it? ... as in, if
I've got the pathway tracing correct, it just goes to the infiniband
device and gets mapped there.  If there's a mask mismatch (very
unlikely, I know) we get a very subtle and hard to trace error.

James