ACPI : Create "idle=nomwait" bootparam

"idle=nomwait" disables the use of the MWAIT
instruction from both C1 (C1_FFH) and deeper (C2C3_FFH)
C-states.

When MWAIT is unavailable, the BIOS and OS generally
negotiate to use the HALT instruction for C1,
and use IO accesses for deeper C-states.

This option is useful for power and performance
comparisons, and also to work around BIOS bugs
where broken MWAIT support is advertised.

http://bugzilla.kernel.org/show_bug.cgi?id=10807
http://bugzilla.kernel.org/show_bug.cgi?id=10914

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Li Shaohua <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>

2 files changed, 13 insertions, 0 deletions

diff --git a/arch/ia64/kernel/process.c b/arch/ia64/kernel/process.c
index 612b3c4a0603..3ab8373103ec 100644
--- a/arch/ia64/kernel/process.c
+++ b/arch/ia64/kernel/process.c
@@ -57,6 +57,8 @@ unsigned long boot_option_idle_override = 0;
 EXPORT_SYMBOL(boot_option_idle_override);
 unsigned long idle_halt;
 EXPORT_SYMBOL(idle_halt);
+unsigned long idle_nomwait;
+EXPORT_SYMBOL(idle_nomwait);
 
 void
 ia64_do_show_stack (struct unw_frame_info *info, void *arg)
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 7fc729498760..4d629c62f4f8 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -11,6 +11,8 @@
 
 unsigned long idle_halt;
 EXPORT_SYMBOL(idle_halt);
+unsigned long idle_nomwait;
+EXPORT_SYMBOL(idle_nomwait);
 
 struct kmem_cache *task_xstate_cachep;
 
@@ -340,6 +342,15 @@ static int __init idle_setup(char *str)
                 pm_idle = default_idle;
                 idle_halt = 1;
                 return 0;
+        } else if (!strcmp(str, "nomwait")) {
+                /*
+                 * If the boot option of "idle=nomwait" is added,
+                 * it means that mwait will be disabled for CPU C2/C3
+                 * states. In such case it won't touch the variable
+                 * of boot_option_idle_override.
+                 */
+                idle_nomwait = 1;
+                return 0;
         } else
                 return -1;