Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752115AbbEEApN (ORCPT ); Mon, 4 May 2015 20:45:13 -0400 Received: from cantor2.suse.de ([195.135.220.15]:56288 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751973AbbEEApJ (ORCPT ); Mon, 4 May 2015 20:45:09 -0400 Date: Tue, 5 May 2015 02:45:06 +0200 From: "Luis R. Rodriguez" To: Randy Dunlap Cc: "Luis R. Rodriguez" , mingo@elte.hu, tglx@linutronix.de, hpa@zytor.com, bp@suse.de, plagnioj@jcrosoft.com, tomi.valkeinen@ti.com, daniel.vetter@intel.com, airlied@linux.ie, dledford@redhat.com, awalls@md.metrocast.net, syrjala@sci.fi, luto@amacapital.net, mst@redhat.com, cocci@systeme.lip6.fr, linux-kernel@vger.kernel.org, Toshi Kani , Jonathan Corbet , Dave Hansen , Suresh Siddha , Juergen Gross , Daniel Vetter , Dave Airlie , Antonino Daplas , Mel Gorman , Vlastimil Babka , Davidlohr Bueso , linux-fbdev@vger.kernel.org Subject: Re: [PATCH v4 2/6] x86: document WC MTRR effects on PAT / non-PAT pages Message-ID: <20150505004506.GI5622@wotan.suse.de> References: <1430343851-967-1-git-send-email-mcgrof@do-not-panic.com> <1430343851-967-3-git-send-email-mcgrof@do-not-panic.com> <5542A628.8050705@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5542A628.8050705@infradead.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: 3904 Lines: 82 On Thu, Apr 30, 2015 at 03:01:12PM -0700, Randy Dunlap wrote: > On 04/29/15 14:44, Luis R. Rodriguez wrote: > > From: "Luis R. Rodriguez" > > diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt > > index cf08c9f..7e183e3 100644 > > --- a/Documentation/x86/pat.txt > > +++ b/Documentation/x86/pat.txt > > @@ -102,7 +104,43 @@ wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc() > > as step 0 above and also track the usage of those pages and use set_memory_wb() > > before the page is freed to free pool. > > > > - > > +MTRR effects on PAT / non-PAT systems > > +------------------------------------- > > + > > +The following table provides the effects of using write-combining MTRRs when > > +using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally > > +mtrr_add() usage will be phased in favor of arch_phys_wc_add() which will > > +be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add() > > +is made should already have be ioremap'd with write-combining page attributes > > +or PAT entries, this can be done by using ioremap_wc() / or respective helpers. > > +Devices which combine areas of IO memory desired to remain uncachable with > > I would spell it uncacheable. In kernel Documentation/, grep uncacheable finds > 14 hits vs. 6 hits for uncachable. No big deal. > > > +areas where write-combining is desirable and are restricted by the size > > +requirements of MTRRs should consider splitting up their IO memory space > > +cleanly with ioremap_uc() and ioremap_wc() followed by an arch_phys_wc_add() > > +encompassing both regions. Such use is nevertheless heavily discouraged as > > +the effective memory type is considered implementation defined. This strategy > > +should only be used as last resort on devices with size-contrained regions > > size-constrained > > > +where otherwise MTRR write-combining would not be effective. > > + > > +Note that you cannot use set_memory_wc() to override / whitelist IO remapped > > +memory space mapped with ioremap*() calls, set_memory_wc() can only be used > > +on RAM. > > + > > +---------------------------------------------------------------------- > > +MTRR Non-PAT PAT Linux ioremap value Effective memory type > > +---------------------------------------------------------------------- > > + Non-PAT | PAT > > + PAT > > + |PCD > > + ||PWT > > + ||| > > +WC 000 WB _PAGE_CACHE_MODE_WB WC | WC > > +WC 001 WC _PAGE_CACHE_MODE_WC WC* | WC > > +WC 010 UC- _PAGE_CACHE_MODE_UC_MINUS WC* | WC > > +WC 011 UC _PAGE_CACHE_MODE_UC UC | UC > > +---------------------------------------------------------------------- > > + > > +(*) denotes implementation defined and is discouraged > > > > Notes: > > > > diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c > > index ea5f363..12abdbe 100644 > > --- a/arch/x86/kernel/cpu/mtrr/main.c > > +++ b/arch/x86/kernel/cpu/mtrr/main.c > > @@ -538,6 +538,9 @@ EXPORT_SYMBOL(mtrr_del); > > * attempts to add a WC MTRR covering size bytes starting at base and > > * logs an error if this fails. > > * > > + * The caller should expect to need to provide a power of two size on an > > * The called should provide a power of two size on an equivalent > * power of two boundary. > Thanks since Boris took this already I'll let him amend unless he wishes for me to send a new version. 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/