diff options
Diffstat (limited to 'Documentation/x86')
| -rw-r--r-- | Documentation/x86/mtrr.txt | 20 |
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. |