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authorLuis R. Rodriguez <mcgrof@suse.com>2015-08-24 12:13:33 -0700
committerIngo Molnar <mingo@kernel.org>2015-08-28 10:09:28 +0200
commit2baa891e42d84159b693eadd44f6fe1486285bdc (patch)
tree260ea29662eac3459c98a6d1211f1cd9be2c7030 /Documentation/x86
parentee03c5868e74847ba0bd1e6dccbbe26e6504466d (diff)
x86/mm/mtrr: Remove kernel internal MTRR interfaces: unexport mtrr_add() and mtrr_del()
The effort to replace mtrr_add() with architecture agnostic arch_phys_wc_add() is complete, this will ensure write-combining implementations (PAT on x86) is taken advantage instead of using MTRR. With the effort done now, hide direct MTRR access for drivers. The legacy user-space /proc/mtrr ABI is not affected. Update x86 documentation on MTRR to reflect the completion of the phasing out of direct access to MTRR, also add a note on platform firmware code use of MTRRs based on the obituary discussion of MTRRs on Linux [0]. [0] http://lkml.kernel.org/r/1438991330.3109.196.camel@hp.com Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com> Cc: <syrjala@sci.fi> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Andy Walls <awalls@md.metrocast.net> Cc: Antonino Daplas <adaplas@gmail.com> Cc: Borislav Petkov <bp@suse.de> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Dave Airlie <airlied@redhat.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Davidlohr Bueso <dbueso@suse.de> Cc: Doug Ledford <dledford@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com> Cc: Juergen Gross <jgross@suse.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mel Gorman <mgorman@suse.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Suresh Siddha <sbsiddha@gmail.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tomi Valkeinen <tomi.valkeinen@ti.com> Cc: Toshi Kani <toshi.kani@hp.com> Cc: Ville Syrjälä <syrjala@sci.fi> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: airlied@linux.ie Cc: benh@kernel.crashing.org Cc: bhelgaas@google.com Cc: dan.j.williams@intel.com Cc: konrad.wilk@oracle.com Cc: linux-fbdev@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: linux-media@vger.kernel.org Cc: mst@redhat.com Cc: netdev@vger.kernel.org Cc: vinod.koul@intel.com Cc: xen-devel@lists.xensource.com Link: http://lkml.kernel.org/r/1440443613-13696-12-git-send-email-mcgrof@do-not-panic.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'Documentation/x86')
-rw-r--r--Documentation/x86/mtrr.txt20
1 files changed, 16 insertions, 4 deletions
diff --git a/Documentation/x86/mtrr.txt b/Documentation/x86/mtrr.txt
index 860bc3adc223..dc3e703913ac 100644
--- a/Documentation/x86/mtrr.txt
+++ b/Documentation/x86/mtrr.txt
@@ -6,10 +6,22 @@ Luis R. Rodriguez <mcgrof@do-not-panic.com> - 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.
+MTRR use is replaced on modern x86 hardware with PAT. Direct MTRR use by
+drivers on Linux is now completely phased out, device drivers should use
+arch_phys_wc_add() in combination with ioremap_wc() to make MTRR effective on
+non-PAT systems while a no-op but equally effective on PAT enabled systems.
+
+Even if Linux does not use MTRRs directly, some x86 platform firmware may still
+set up MTRRs early before booting the OS. They do this as some platform
+firmware may still have implemented access to MTRRs which would be controlled
+and handled by the platform firmware directly. An example of platform use of
+MTRRs is through the use of SMI handlers, one case could be for fan control,
+the platform code would need uncachable access to some of its fan control
+registers. Such platform access does not need any Operating System MTRR code in
+place other than mtrr_type_lookup() to ensure any OS specific mapping requests
+are aligned with platform MTRR setup. If MTRRs are only set up by the platform
+firmware code though and the OS does not make any specific MTRR mapping
+requests mtrr_type_lookup() should always return MTRR_TYPE_INVALID.
For details refer to Documentation/x86/pat.txt.