[PATCH] ppc64: enable POWER5 low power mode in idle loop

Now that we understand (and have fixed) the problem with using low power mode
in the idle loop, lets enable it.  It should save a fair amount of power.

(The problem was that our exceptions were inheriting the low power mode and so
were executing at a fraction of the normal cpu issue rate.  We fixed it by
always bumping our priority to medium at the start of every exception).
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/ppc64/kernel/idle.c

@@ -142,7 +142,12 @@ int default_idle(void)
 
 			while (!need_resched() && !cpu_is_offline(cpu)) {
 				barrier();
+				/*
+				 * Go into low thread priority and possibly
+				 * low power mode.
+				 */
 				HMT_low();
+				HMT_very_low();
 			}
 
 			HMT_medium();
@@ -184,18 +189,18 @@ int dedicated_idle(void)
 			start_snooze = __get_tb() +
 				*smt_snooze_delay * tb_ticks_per_usec;
 			while (!need_resched() && !cpu_is_offline(cpu)) {
-				/* need_resched could be 1 or 0 at this 
-				 * point.  If it is 0, set it to 0, so
-				 * an IPI/Prod is sent.  If it is 1, keep
-				 * it that way & schedule work.
+				/*
+				 * Go into low thread priority and possibly
+				 * low power mode.
 				 */
+				HMT_low();
+				HMT_very_low();
+
 				if (*smt_snooze_delay == 0 ||
-				    __get_tb() < start_snooze) {
-					HMT_low(); /* Low thread priority */
+				    __get_tb() < start_snooze)
 					continue;
-				}
 
-				HMT_very_low(); /* Low power mode */
+				HMT_medium();
 
 				if (!(ppaca->lppaca.xIdle)) {
 					/* Indicate we are no longer polling for
@@ -210,7 +215,6 @@ int dedicated_idle(void)
 						break;
 					}
 
-					/* DRENG: Go HMT_medium here ? */
 					local_irq_disable(); 
 
 					/* SMT dynamic mode.  Cede will result