diff options
Diffstat (limited to 'Documentation/x86/pat.txt')
| -rw-r--r-- | Documentation/x86/pat.txt | 48 |
1 files changed, 43 insertions, 5 deletions
diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt index cf08c9fff3cd..54944c71b819 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 |