ACPI: Create "idle=halt" bootparam

"idle=halt" limits the idle loop to using
the halt instruction.  No MWAIT, no IO accesses,
no C-states deeper than C1.

If something is broken in the idle code,
"idle=halt" is a less severe workaround
than "idle=poll" which disables all power savings.

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

arch/x86/kernel/process.c

@@ -7,6 +7,10 @@
 #include <linux/module.h>
 #include <linux/pm.h>
 #include <linux/clockchips.h>
+#include <asm/system.h>
+
+unsigned long idle_halt;
+EXPORT_SYMBOL(idle_halt);
 
 struct kmem_cache *task_xstate_cachep;
 
@@ -325,7 +329,18 @@ static int __init idle_setup(char *str)
 		pm_idle = poll_idle;
 	} else if (!strcmp(str, "mwait"))
 		force_mwait = 1;
-	else
+	else if (!strcmp(str, "halt")) {
+		/*
+		 * When the boot option of idle=halt is added, halt is
+		 * forced to be used for CPU idle. In such case CPU C2/C3
+		 * won't be used again.
+		 * To continue to load the CPU idle driver, don't touch
+		 * the boot_option_idle_override.
+		 */
+		pm_idle = default_idle;
+		idle_halt = 1;
+		return 0;
+	} else
 		return -1;
 
 	boot_option_idle_override = 1;