From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Subject: Re: PATCH: Further aacraid work Date: Fri, 18 Jun 2004 10:19:27 -0500 Sender: linux-kernel-owner@vger.kernel.org Message-ID: <1087571966.8209.273.camel@gaston> References: <286GI-5y3-11@gated-at.bofh.it> <286Qp-5EU-19@gated-at.bofh.it> <20040617205414.GE8705@devserv.devel.redhat.com> <20040617212559.GA71701@colin2.muc.de> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20040617212559.GA71701@colin2.muc.de> To: Andi Kleen Cc: Alan Cox , Anton Blanchard , mark_salyzyn@adaptec.com, Christoph Hellwig , Linux Kernel list , linux-scsi@vger.kernel.org List-Id: linux-scsi@vger.kernel.org On Thu, 2004-06-17 at 16:25, Andi Kleen wrote: > On Thu, Jun 17, 2004 at 04:54:14PM -0400, Alan Cox wrote: > > I would rather see it below the I/O layer for things like AMD64. The > > reason I say this is that many drivers would suffer from iommu merging not > > gain, and others may have limits. > > > > Something like > > > > new_sglist = sg_squash(old_sglist, [target max segments], [max per seg]) > > > > could be used by drivers when appropriate to hand back a better sg list > > (or if not possible the existing one). That would put control rather closer > > to the driver. > > My understanding was that it was too late in the driver because the SG lists > are already sized, because higher layer manage this. That is why > the BIO_VMERGE_BOUNDARY define is checked by BIO, not the driver. > > The input of sg_squash should not be an already mapped list > (that would be too costly) better would be probably > a pci_map_sg_merge() with hints that tries to merge and other > than that works like normal pci_map_sg() Well, that's why we don't enable BIO_VMERGE_BOUNDARY nor any of the merging at the BIO level, at least we didn't on ppc64 when I last worked on the code. We let the BIO generate things that will always fit. We just have pci_map_* do merging on a "best it can" basis. Most of the time, it does end up merging a lot. I don't think any driver control would help much here ... Ben.