Re: Concerns about "mpt2sas: Added Reply Descriptor Post Queue (RDPQ) Array support"

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2015-02-19 at 21:45 -0800, James Bottomley wrote:
> Ben, this is legal by design.  It was specifically designed for the
> aic79xx SCSI card, but can be used for a variety of other reasons.  The
> aic79xx hardware problem was that the DMA engine could address the whole
> of memory (it had two address modes, a 39 bit one and a 64 bit one) but
> the script engine that runs the mailboxes only had a 32 bit activation
> register (the activating write points at the physical address of the
> script to begin executing).  This meant that the scripts that run in
> memory had to be in the first 4GB of physical memory, hence the split
> mask.  The DMA mask specifies that the card can transfer from anywhere
> in physical memory, but the consistent_dma_mask says that the consistent
> allocation used to get scripts memory must come from the lower 4GB.

Right, ok, it looks like it's easy enough to support with ZONE_DMA32, I'm
testing patches to create it unconditionally on ppc64 (it used to depend
on us using swiotlb on embedded platforms) and I'll shoot that upstream
if it passes.

Ben.