Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753220AbbGBStR (ORCPT ); Thu, 2 Jul 2015 14:49:17 -0400 Received: from cantor2.suse.de ([195.135.220.15]:60519 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754155AbbGBStK (ORCPT ); Thu, 2 Jul 2015 14:49:10 -0400 Date: Thu, 2 Jul 2015 20:49:04 +0200 From: "Luis R. Rodriguez" To: Benjamin Herrenschmidt Cc: Casey Leedom , Arnd Bergmann , "Michael S. Tsirkin" , Bjorn Helgaas , Toshi Kani , Andy Lutomirski , Juergen Gross , Tomi Valkeinen , "linux-pci@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "xen-devel@lists.xensource.com" , linux-fbdev , Suresh Siddha , Ingo Molnar , Thomas Gleixner , Daniel Vetter , Dave Airlie , Antonino Daplas , Jean-Christophe Plagniol-Villard , Dave Hansen , "venkatesh.pallipadi@intel.com" , Stefan Bader , "ville.syrjala@linux.intel.com" , David Vrabel , Jan Beulich , Roger Pau =?iso-8859-1?Q?Monn=E9?= Subject: Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants Message-ID: <20150702184904.GA7021@wotan.suse.de> References: <1434751712-24333-1-git-send-email-mcgrof@do-not-panic.com> <1435189081.3790.24.camel@kernel.crashing.org> <4985EFDD773FCB459EF7915D2A3621ADC02F10@nice.asicdesigners.com> <6806026.xb91q6Ad7G@wuerfel> <4985EFDD773FCB459EF7915D2A3621ADC031F8@nice.asicdesigners.com> <1435284123.3822.24.camel@kernel.crashing.org> <4985EFDD773FCB459EF7915D2A3621ADC03621@nice.asicdesigners.com> <1435356048.26815.9.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1435356048.26815.9.camel@kernel.crashing.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2124 Lines: 43 On Sat, Jun 27, 2015 at 08:00:48AM +1000, Benjamin Herrenschmidt wrote: > On Fri, 2015-06-26 at 16:24 +0000, Casey Leedom wrote: > > Thanks for looking into this Ben. As it stands now, it seems as > > if Write Combined mappings simply aren't supported and/or all > > driver writers trying to utilize Write Combined mappings have to > > "hand roll" their own solutions which really isn't supportable. > > > > One thing that might be considered is simply to treat the desire > > to utilize the Write Combining hardware as a separate issue and > > develop writel_wc(), writeq_wc(), etc. That seems rather sloppy and cumbersome, its best to just provide the infrastructure for initial mapping for an area and let the hardware do it for you. With the current design drivers would just use regular read/write on all areas and the only thing that will set them apart will be the mapping. With what you propose we'd end up having to shift a whole bunch of reads/writes for just the purpose of adding WC to an area that didn't have wc before. That's a waste of code and time. > > Those could be defined > > as legal only for Write Combined Mappings and would "do the > > right thing" for each architecture. Yuck. > The question then is what is "the right thing". In the powerpc case, > we'll have a non-garded mapping, which means we also get no ordering > between load and stores. I don't follow, you *ordering* between load and stores for WC? We should not need that for WC, its why WC is used for only very specific things such as framebuffer and PIO (which BTw I still don't quite get all this use case for infiniband to be honest, and I will note I do see some proprietary hardware extensions like bursts but nothing covering all this in a general doc, I think I think it all just has to do that this is a hardware hack in reality, which we sell as a feature). Luis -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/