diff options
| -rw-r--r-- | Documentation/x86/mtrr.txt | 20 | ||||
| -rw-r--r-- | arch/x86/kernel/cpu/mtrr/main.c | 2 |
2 files changed, 16 insertions, 6 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. diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c index e7ed0d8ebacb..f891b4750f04 100644 --- a/arch/x86/kernel/cpu/mtrr/main.c +++ b/arch/x86/kernel/cpu/mtrr/main.c @@ -448,7 +448,6 @@ int mtrr_add(unsigned long base, unsigned long size, unsigned int type, return mtrr_add_page(base >> PAGE_SHIFT, size >> PAGE_SHIFT, type, increment); } -EXPORT_SYMBOL(mtrr_add); /** * mtrr_del_page - delete a memory type region @@ -537,7 +536,6 @@ int mtrr_del(int reg, unsigned long base, unsigned long size) return -EINVAL; return mtrr_del_page(reg, base >> PAGE_SHIFT, size >> PAGE_SHIFT); } -EXPORT_SYMBOL(mtrr_del); /** * arch_phys_wc_add - add a WC MTRR and handle errors if PAT is unavailable |