Re: libata .sg_tablesize: why always dividing by 2 ?

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

On Wed, 2008-02-27 at 08:50 +1100, Benjamin Herrenschmidt wrote:
> On Tue, 2008-02-26 at 11:51 -0500, Mark Lord wrote:
> > > That's interesting.  Can you point us to the exact file::lines where
> > > this happens?  It would be good to ensure that this gets fixed.
> > > 
> > > I'm copying Fujita Tomonori on this thread now -- he's the dude
> > who's
> > > trying to sort out the IOMMU mess.
> > ..
> > 
> > Mmm.. looks like ppc is already fixed in mainline,
> > commit 740c3ce66700640a6e6136ff679b067e92125794
> 
> No, I only fixed alignent up to PAGE_SIZE, not above.
> 
> Our iommu can use smaller than PAGE_SIZE allocations when PAGE_SIZE is
> 64k (it can use 4k), and that helps a lot with networking to avoid
> filling up pSeries small iommu's too fast. Unfortunately, that meant we
> used to not even get PAGE_SIZE alignment for some cases. The workaround
> fixes that but does not provide natural size alignment.
> 
> If we were to provide size based alignment of sg requests, fragmentation
> would become so bad we would basically end up failing a large amount of
> iommu allocations on servers.

Ben, he just quoted the wrong patch ... that was the segment length
limit patch.  The boundary alignment patch is this one:

fb3475e9b6bfa666107512fbd6006c26014f04b8

And it does alter the ppc iommu to respect the dma_boundary, so all of
the mechanics for removing the ppc special casing in libata/ide is
upstream.

James