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-rw-r--r--Documentation/x86/pat.txt48
1 files changed, 43 insertions, 5 deletions
diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt
index cf08c9fff..54944c71b 100644
--- a/Documentation/x86/pat.txt
+++ b/Documentation/x86/pat.txt
@@ -12,7 +12,7 @@ virtual addresses.
PAT allows for different types of memory attributes. The most commonly used
ones that will be supported at this time are Write-back, Uncached,
-Write-combined and Uncached Minus.
+Write-combined, Write-through and Uncached Minus.
PAT APIs
@@ -34,16 +34,23 @@ ioremap | -- | UC- | UC- |
| | | |
ioremap_cache | -- | WB | WB |
| | | |
+ioremap_uc | -- | UC | UC |
+ | | | |
ioremap_nocache | -- | UC- | UC- |
| | | |
ioremap_wc | -- | -- | WC |
| | | |
+ioremap_wt | -- | -- | WT |
+ | | | |
set_memory_uc | UC- | -- | -- |
set_memory_wb | | | |
| | | |
set_memory_wc | WC | -- | -- |
set_memory_wb | | | |
| | | |
+set_memory_wt | WT | -- | -- |
+ set_memory_wb | | | |
+ | | | |
pci sysfs resource | -- | -- | UC- |
| | | |
pci sysfs resource_wc | -- | -- | WC |
@@ -102,7 +109,38 @@ 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:
@@ -115,8 +153,8 @@ can be more restrictive, in case of any existing aliasing for that address.
For example: If there is an existing uncached mapping, a new ioremap_wc can
return uncached mapping in place of write-combine requested.
-set_memory_[uc|wc] and set_memory_wb should be used in pairs, where driver will
-first make a region uc or wc and switch it back to wb after use.
+set_memory_[uc|wc|wt] and set_memory_wb should be used in pairs, where driver
+will first make a region uc, wc or wt and switch it back to wb after use.
Over time writes to /proc/mtrr will be deprecated in favor of using PAT based
interfaces. Users writing to /proc/mtrr are suggested to use above interfaces.
@@ -124,7 +162,7 @@ interfaces. Users writing to /proc/mtrr are suggested to use above interfaces.
Drivers should use ioremap_[uc|wc] to access PCI BARs with [uc|wc] access
types.
-Drivers should use set_memory_[uc|wc] to set access type for RAM ranges.
+Drivers should use set_memory_[uc|wc|wt] to set access type for RAM ranges.
PAT debugging