From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: RFC on writel and writel_relaxed Date: Wed, 28 Mar 2018 09:11:10 +0200 Message-ID: References: <1521854626.16434.359.camel@kernel.crashing.org> <58ce5b83f40f4775bec1be8db66adb0d@AcuMS.aculab.com> <20180326165425.GA15554@ziepe.ca> <20180326202545.GB15554@ziepe.ca> <20180326210951.GD15554@ziepe.ca> <1522101616.7364.13.camel@kernel.crashing.org> <1e077f6a-90b6-cce9-6f0f-a8c003fec850@codeaurora.org> <20180327151029.GB17494@arm.com> <1522186396.7364.61.camel@kernel.crashing.org> <1522198981.7364.81.camel@kernel.crashing.org> <1522211620.7364.94.camel@kernel.crashing.org> <1522219376.7364.109.camel@kernel.crashing.org> <1522220165.7364.110.camel@kernel.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <1522220165.7364.110.camel@kernel.crashing.org> Sender: netdev-owner@vger.kernel.org To: Benjamin Herrenschmidt Cc: Linus Torvalds , Alexander Duyck , Will Deacon , Sinan Kaya , Jason Gunthorpe , David Laight , Oliver , "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" , "linux-rdma@vger.kernel.org" , "Paul E. McKenney" , "netdev@vger.kernel.org" List-Id: linux-rdma@vger.kernel.org On Wed, Mar 28, 2018 at 8:56 AM, Benjamin Herrenschmidt wrote: > On Wed, 2018-03-28 at 06:53 +0000, Linus Torvalds wrote: >> On Tue, Mar 27, 2018, 20:43 Benjamin Herrenschmidt wrote: >> That's why in/out were *so* slow, and why nobody uses them any more >> (well, the address size limitations and the lack of any remapping of >> the address obviously also are a reason). > > All true indeed, though a lot of other archs never quite made them > fully synchronous, which was another can of worms ... oh well. Many architectures have no way of providing PCI compliant semantics for outb, as their instruction set and/or bus interconnect lacks a method of waiting for completion of an outb. In practice, it doesn't seem to matter for any of the devices one would encounter these days: very few use I/O space, and those that do don't actually rely on the strict ordering. Some architectures (in particular s390, but I remember seeing the same thing elsewhere) explicitly disallow I/O space access on PCI because of this. On ARM, the typical PCI implementations have other problems that are worse than this one, so most drivers are fine with the almost-working semantics. Arnd