[PATCH v4 2/6] x86: document WC MTRR effects on PAT / non-PAT pages

From: Luis R. Rodriguez
Date: Wed Apr 29 2015 - 17:50:52 EST

From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>

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 <toshi.kani@xxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Suresh Siddha <sbsiddha@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxxx>
Cc: Antonino Daplas <adaplas@xxxxxxxxx>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@xxxxxxxxxxxx>
Cc: Tomi Valkeinen <tomi.valkeinen@xxxxxx>
Cc: Ville SyrjÃlà <syrjala@xxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: linux-fbdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
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(-)

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
+Richard Gooch <rgooch@xxxxxxxxxxxxx> - 3 Jun 1999
+Luis R. Rodriguez <mcgrof@xxxxxxxxxxxxxxxx> - 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..7e183e3 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
+ |PCD
+ ||PWT
+ |||
+(*) denotes implementation defined and is discouraged


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.

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