From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Subject: Re: Concerns about "mpt2sas: Added Reply Descriptor Post Queue (RDPQ) Array support" Date: Fri, 20 Feb 2015 18:19:21 +1100 Message-ID: <1424416761.27448.37.camel@kernel.crashing.org> References: <1424408500.27448.25.camel@kernel.crashing.org> <1424408769.27448.28.camel@kernel.crashing.org> <1424409761.27448.32.camel@kernel.crashing.org> <1424411110.16222.5.camel@HansenPartnership.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1424411110.16222.5.camel@HansenPartnership.com> Sender: linux-kernel-owner@vger.kernel.org To: James Bottomley Cc: Sreekanth Reddy , "Martin K. Petersen" , Linux Kernel Mailing List , scsi , Christoph Hellwig List-Id: linux-scsi@vger.kernel.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.