From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [PATCH v4 4/6] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs Date: Thu, 22 Mar 2018 15:25:10 -0600 Message-ID: <20180322212510.GE9469@ziepe.ca> References: <1521514068-8856-5-git-send-email-okaya@codeaurora.org> <201803221430.P43GJl9U%fengguang.wu@intel.com> <3664b253c730dbf83f4528acaedb3a88@codeaurora.org> <3e9c006e4541acbce11743dbda553e84@codeaurora.org> <03d201d3c1eb$b71fb460$255f1d20$@opengridcomputing.com> <83484a3f-d3f7-d763-e4f8-e4fec3bb8cc2@codeaurora.org> <52cbc9d7-5a6b-5c8b-b930-058f5be62079@opengridcomputing.com> <20180322201649.GC9469@ziepe.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Casey Leedom Cc: SWise OGC , Sinan Kaya , 'kbuild test robot' , "kbuild-all@01.org" , "linux-rdma@vger.kernel.org" , "timur@codeaurora.org" , "sulrich@codeaurora.org" , "linux-arm-msm@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , Steve Wise , 'Doug Ledford' , "linux-kernel@vger.kernel.org" , Michael Werner List-Id: linux-arm-msm@vger.kernel.org On Thu, Mar 22, 2018 at 08:45:11PM +0000, Casey Leedom wrote: > I'm guessing~ that this line in the documentation ~may~ imply the GCC > ordering: > > ... Note that relaxed accesses to > the same peripheral are guaranteed to be ordered with respect to each > other. ... An arch can't guarentee "ordered with respect to each other" without preventing the compiler from re-ordering, so yes, any correct implementation of writel_relaxed must prevent compiler re-ordering. eg with volatile or a compiler barrier, or whatever. > In any case, we really only have a few places where we (the various Chelsio > drivers) need to worry about this: the "Fast Paths" where we have a lot of > I/O to the device. I think we should leave everything else alone. Yes. Jason From mboxrd@z Thu Jan 1 00:00:00 1970 From: jgg@ziepe.ca (Jason Gunthorpe) Date: Thu, 22 Mar 2018 15:25:10 -0600 Subject: [PATCH v4 4/6] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs In-Reply-To: References: <1521514068-8856-5-git-send-email-okaya@codeaurora.org> <201803221430.P43GJl9U%fengguang.wu@intel.com> <3664b253c730dbf83f4528acaedb3a88@codeaurora.org> <3e9c006e4541acbce11743dbda553e84@codeaurora.org> <03d201d3c1eb$b71fb460$255f1d20$@opengridcomputing.com> <83484a3f-d3f7-d763-e4f8-e4fec3bb8cc2@codeaurora.org> <52cbc9d7-5a6b-5c8b-b930-058f5be62079@opengridcomputing.com> <20180322201649.GC9469@ziepe.ca> Message-ID: <20180322212510.GE9469@ziepe.ca> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Mar 22, 2018 at 08:45:11PM +0000, Casey Leedom wrote: > I'm guessing~ that this line in the documentation ~may~ imply the GCC > ordering: > > ... Note that relaxed accesses to > the same peripheral are guaranteed to be ordered with respect to each > other. ... An arch can't guarentee "ordered with respect to each other" without preventing the compiler from re-ordering, so yes, any correct implementation of writel_relaxed must prevent compiler re-ordering. eg with volatile or a compiler barrier, or whatever. > In any case, we really only have a few places where we (the various Chelsio > drivers) need to worry about this: the "Fast Paths" where we have a lot of > I/O to the device. I think we should leave everything else alone. Yes. Jason