Re: Concerns about "mpt2sas: Added Reply Descriptor Post Queue (RDPQ) Array support"
From: Benjamin Herrenschmidt
Date: Fri Feb 20 2015 - 02:19:38 EST
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.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/