From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v1 06/47] mtrr: add __arch_phys_wc_add() Date: Fri, 20 Mar 2015 16:48:46 -0700 Message-ID: References: <1426893517-2511-1-git-send-email-mcgrof@do-not-panic.com> <1426893517-2511-7-git-send-email-mcgrof@do-not-panic.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1YZ6f7-0004mj-E2 for xen-devel@lists.xenproject.org; Fri, 20 Mar 2015 23:49:09 +0000 Received: by labjg1 with SMTP id jg1so98417753lab.2 for ; Fri, 20 Mar 2015 16:49:06 -0700 (PDT) In-Reply-To: <1426893517-2511-7-git-send-email-mcgrof@do-not-panic.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: "Luis R. Rodriguez" Cc: Juergen Gross , Linux Fbdev development list , X86 ML , Suresh Siddha , Antonino Daplas , Daniel Vetter , "Luis R. Rodriguez" , venkatesh.pallipadi@intel.com, "linux-kernel@vger.kernel.org" , "xen-devel@lists.xenproject.org" , Ingo Molnar , Tomi Valkeinen , Jan Beulich , "H. Peter Anvin" , Dave Airlie , Thomas Gleixner , Borislav Petkov , Jean-Christophe Plagniol-Villard , Ingo Molnar List-Id: xen-devel@lists.xenproject.org On Fri, Mar 20, 2015 at 4:17 PM, Luis R. Rodriguez wrote: > From: "Luis R. Rodriguez" > > Ideally on systems using PAT we can expect a swift > transition away from MTRR. There can be a few exceptions > to this, one is where device drivers are known to exist > on PATs with errata, another situation is observed on > old device drivers where devices had combined MMIO > register access with whatever area they typically > later wanted to end up using MTRR for on the same > PCI BAR. This situation can still be addressed by > splitting up ioremap'd PCI BAR into two ioremap'd > calls, one for MMIO registers, and another for whatever > is desirable for write-combining -- in order to > accomplish this though quite a bit of driver > restructuring is required. > > Device drivers which are known to require large > amount of re-work in order to split ioremap'd areas > can use __arch_phys_wc_add() to avoid regressions > when PAT is enabled. > > For a good example driver where things are neatly > split up on a PCI BAR refer the infiniband qib > driver. For a good example of a driver where good > amount of work is required refer to the infiniband > ipath driver. > > This is *only* a transitive API -- and as such no new > drivers are ever expected to use this. What's the exact layout that this helps? I'm sceptical that this can ever be correct. Is there some awful driver that has a large ioremap that's supposed to contain multiple different memtypes? If so, can we ioremap + set_page_xyz instead? --Andy