[ACPI] Allow limiting idle C-States.

Useful to workaround C3 ipw2100 packet loss,
reducing noise or boot issues on some models.
http://bugme.osdl.org/show_bug.cgi?id=3549

For static processor driver, boot cmdline:
processor.acpi_cstate_limit=2

For processor module, /etc/modprobe.conf:
options processor acpi_cstate_limit=2

For manual processor module load:
# modprobe processor acpi_cstate_limit=2

From kernel or kernel module:
#include <linux/acpi.h>
acpi_set_cstate_limit(2);

Inspired by patches from Jos Delbar and Andi Kleen
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/osl.c

@@ -1078,3 +1078,12 @@ acpi_leave_gpes_disabled_setup(char *str)
 
 __setup("acpi_leave_gpes_disabled", acpi_leave_gpes_disabled_setup);
 
+/*
+ * acpi_cstate_limit is defined in the base kernel so modules can
+ * change it w/o depending on the state of the processor module.
+ */
+unsigned int acpi_cstate_limit = ACPI_C_STATES_MAX;
+
+
+EXPORT_SYMBOL(acpi_cstate_limit);
+

drivers/acpi/processor.c

@@ -459,8 +459,9 @@ acpi_processor_idle (void)
 	 * Track the number of longs (time asleep is greater than threshold)
 	 * and promote when the count threshold is reached.  Note that bus
 	 * mastering activity may prevent promotions.
+	 * Do not promote above acpi_cstate_limit.
 	 */
-	if (cx->promotion.state) {
+	if (cx->promotion.state && (cx->promotion.state <= acpi_cstate_limit)) {
 		if (sleep_ticks > cx->promotion.threshold.ticks) {
 			cx->promotion.count++;
  			cx->demotion.count = 0;
@@ -497,6 +498,13 @@ acpi_processor_idle (void)
 	}
 
 end:
+	/*
+	 * Demote if current state exceeds acpi_cstate_limit
+	 */
+	if (pr->power.state > acpi_cstate_limit) {
+		next_state = acpi_cstate_limit;
+	}
+
 	/*
 	 * New Cx State?
 	 * -------------
@@ -2441,5 +2449,6 @@ acpi_processor_exit (void)
 
 module_init(acpi_processor_init);
 module_exit(acpi_processor_exit);
+module_param_named(acpi_cstate_limit, acpi_cstate_limit, uint, 0);
 
 EXPORT_SYMBOL(acpi_processor_set_thermal_limit);

include/linux/acpi.h

@@ -471,4 +471,30 @@ static inline int acpi_blacklisted(void)
 
 #endif /*!CONFIG_ACPI_INTERPRETER*/
 
+#define	ACPI_CSTATE_LIMIT_DEFINED	/* for driver builds */
+#ifdef	CONFIG_ACPI
+
+/*
+ * Set highest legal C-state
+ * 0: C0 okay, but not C1
+ * 1: C1 okay, but not C2
+ * 2: C2 okay, but not C3 etc.
+ */
+
+extern unsigned int acpi_cstate_limit;
+
+static inline unsigned int acpi_get_cstate_limit(void)
+{
+	return acpi_cstate_limit;
+}
+static inline void acpi_set_cstate_limit(unsigned int new_limit)
+{
+	acpi_cstate_limit = new_limit;
+	return;
+}
+#else
+static inline unsigned int acpi_get_cstate_limit(void) { return 0; }
+static inline void acpi_set_cstate_limit(unsigned int new_limit) { return; }
+#endif
+
 #endif /*_LINUX_ACPI_H*/