[v3,2/6] x86: document WC MTRR effects on PAT / non-PAT pages

Commit Message

Luis R. Rodriguez April 21, 2015, 8:26 p.m. UTC

From: "Luis R. Rodriguez" <[e-mail]>

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: Toshi Kani <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Suresh Siddha <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Antonino Daplas <[e-mail]>
Cc: Jean-Christophe Plagniol-Villard <[e-mail]>
Cc: Tomi Valkeinen <[e-mail]>
Cc: Ville Syrjälä <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Luis R. Rodriguez <[e-mail]>
---
 Documentation/x86/mtrr.txt      | 18 +++++++++++++++---
 Documentation/x86/pat.txt       | 40 +++++++++++++++++++++++++++++++++++++++-
 arch/x86/kernel/cpu/mtrr/main.c |  3 +++
 3 files changed, 57 insertions(+), 4 deletions(-)

Patch

diff --git a/Documentation/x86/mtrr.txt b/Documentation/x86/mtrr.txt
index cc071dc..a111a6c 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
-<[e-mail]>
+
+Richard Gooch <[e-mail]> - 3 Jun 1999
+Luis R. Rodriguez <[e-mail]> - April 9, 2015
+
+===============================================================================
+Phasing 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..2c27d20 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 +104,43 @@  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
+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
+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*  |   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 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
+ * 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.
  */