Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756341AbbEEHzq (ORCPT ); Tue, 5 May 2015 03:55:46 -0400 Received: from mail-pa0-f47.google.com ([209.85.220.47]:34111 "EHLO mail-pa0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752782AbbEEHzo (ORCPT ); Tue, 5 May 2015 03:55:44 -0400 From: "Luis R. Rodriguez" To: bp@suse.de Cc: mingo@elte.hu, tglx@linutronix.de, hpa@zytor.com, plagnioj@jcrosoft.com, tomi.valkeinen@ti.com, linux-fbdev@vger.kernel.org, luto@amacapital.net, mst@redhat.com, syrjala@sci.fi, linux-kernel@vger.kernel.org, "Luis R. Rodriguez" , Jonathan Corbet , Dave Hansen , Suresh Siddha , Juergen Gross , Daniel Vetter , Dave Airlie , Antonino Daplas , Mel Gorman , Vlastimil Babka , Davidlohr Bueso Subject: [PATCH v5 2/6] x86: document WC MTRR effects on PAT / non-PAT pages Date: Tue, 5 May 2015 00:53:25 -0700 Message-Id: <1430812405-23630-1-git-send-email-mcgrof@do-not-panic.com> X-Mailer: git-send-email 2.1.0 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6558 Lines: 148 From: "Luis R. Rodriguez" As part of the effort to phase out MTRR use document write-combining MTRR effects on pages with different non-PAT page attributes flags and different PAT entry values. Extend arch_phys_wc_add() documentation to clarify power of two sizes / boundary requirements as we phase out mtrr_add() use. Lastly hint towards ioremap_uc() for corner cases on device drivers working with devices with mixed regions where MTRR size requirements would otherwise not enable write-combining effective memory types. Cc: Jonathan Corbet Cc: Dave Hansen Cc: Andy Lutomirski Cc: Suresh Siddha Cc: Ingo Molnar Cc: Thomas Gleixner Cc: Juergen Gross Cc: Daniel Vetter Cc: Dave Airlie Cc: Antonino Daplas Cc: Jean-Christophe Plagniol-Villard Cc: Tomi Valkeinen Cc: Ville Syrjälä Cc: Mel Gorman Cc: Vlastimil Babka Cc: Borislav Petkov Cc: Davidlohr Bueso Cc: linux-fbdev@vger.kernel.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Luis R. Rodriguez --- This v5 goes with the documentation fixes proposed by Randy and Boris. I am only sending the documentation patch with a followup v5 as the other ones have not received any other negative feedback. Would also like to hear back from Ville if he's OK with these changes to atyfb. Documentation/x86/mtrr.txt | 18 +++++++++++++++--- Documentation/x86/pat.txt | 35 ++++++++++++++++++++++++++++++++++- arch/x86/kernel/cpu/mtrr/main.c | 3 +++ 3 files changed, 52 insertions(+), 4 deletions(-) diff --git a/Documentation/x86/mtrr.txt b/Documentation/x86/mtrr.txt index cc071dc..860bc3a 100644 --- a/Documentation/x86/mtrr.txt +++ b/Documentation/x86/mtrr.txt @@ -1,7 +1,19 @@ MTRR (Memory Type Range Register) control -3 Jun 1999 -Richard Gooch - + +Richard Gooch - 3 Jun 1999 +Luis R. Rodriguez - April 9, 2015 + +=============================================================================== +Phasing out MTRR use + +MTRR use is replaced on modern x86 hardware with PAT. Over time the only type +of effective MTRR that is expected to be supported will be for write-combining. +As MTRR use is phased out device drivers should use arch_phys_wc_add() to make +MTRR effective on non-PAT systems while a no-op on PAT enabled systems. + +For details refer to Documentation/x86/pat.txt. + +=============================================================================== On Intel P6 family processors (Pentium Pro, Pentium II and later) the Memory Type Range Registers (MTRRs) may be used to control diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt index cf08c9f..521bd8a 100644 --- a/Documentation/x86/pat.txt +++ b/Documentation/x86/pat.txt @@ -34,6 +34,8 @@ ioremap | -- | UC- | UC- | | | | | ioremap_cache | -- | WB | WB | | | | | +ioremap_uc | -- | UC | UC | + | | | | ioremap_nocache | -- | UC- | UC- | | | | | ioremap_wc | -- | -- | WC | @@ -102,7 +104,38 @@ 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 out 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 been ioremapped with WC attributes or PAT entries, +this can be done by using ioremap_wc() / set_memory_wc(). Devices which +combine areas of IO memory desired to remain uncacheable with areas where +write-combining is desirable should consider use of ioremap_uc() followed by +set_memory_wc() to white-list effective write-combined areas. Such use is +nevertheless discouraged as the effective memory type is considered +implementation defined, yet this strategy can be used as last resort on devices +with size-constrained regions where otherwise MTRR write-combining would +otherwise not be effective. + +---------------------------------------------------------------------- +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* | UC +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 c3ea014..2cc64b8 100644 --- a/arch/x86/kernel/cpu/mtrr/main.c +++ b/arch/x86/kernel/cpu/mtrr/main.c @@ -546,6 +546,9 @@ EXPORT_SYMBOL(mtrr_del); * attempts to add a WC MTRR covering size bytes starting at base and * logs an error if this fails. * + * The called should provide a power of two size on an equivalent + * power of two boundary. + * * Drivers must store the return value to pass to mtrr_del_wc_if_needed, * but drivers should not try to interpret that return value. */ -- 2.3.2.209.gd67f9d5.dirty -- 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/