[PATCH] ia64: Use PAL_HALT_LIGHT in cpu_idle

Here's patches for 2.4 & 2.5 to use PAL_HALT_LIGHT in cpu_idle.
This helps to reduce CPU temp a little on boxes with firmware that
takes advantage of this lower power state.  I've tried this on a
rx2600 (2x900MHz McKinley) and an i2000 (fw 117) and it shows some
benefit.  On McKinley systems, only the very latest PAL from Intel
actually reduces power consumption in the halt_light state.  For HP
rx2600/zx6000/zx2000, this means you need to be running firmware 1.82.

  Rohit Seth, at Intel, has run some benchmarks with this kind of
modification and found the effects of enabling halt_light to fall
within the noise of mosts tests.  I replaced pal_halt(1) in safe_halt
with pal_halt_light() since halt_light is required to be implemented,
but pal_halt(1) is an optional halt state.  I'd be interested to hear
of any measurements anyone does using this, where it works/fails, and
if any benchmarks/applications are impacted.

arch/ia64/Kconfig

@@ -405,6 +405,15 @@ config HUGETLB_PAGE_SIZE_256KB
 
 endchoice
 
+config IA64_PAL_IDLE
+	bool "Use PAL_HALT_LIGHT in idle loop"
+	---help---
+	  Say Y here to enable use of PAL_HALT_LIGHT in the cpu_idle loop.
+	  This allows the CPU to enter a low power state when idle.  You
+	  can enable CONFIG_IA64_PALINFO and check /proc/pal/cpu0/power_info
+	  to see the power consumption and latency for this state.  If you're
+	  unsure your firmware supports it, answer N.
+
 config SMP
 	bool "SMP support"
 	---help---

arch/ia64/kernel/process.c

@@ -169,7 +169,10 @@ do_notify_resume_user (sigset_t *oldset, struct sigscratch *scr, long in_syscall
 void
 default_idle (void)
 {
-	/* may want to do PAL_LIGHT_HALT here... */
+#ifdef CONFIG_IA64_PAL_IDLE
+	if (!need_resched())
+		safe_halt();
+#endif
 }
 
 void __attribute__((noreturn))
@@ -177,6 +180,10 @@ cpu_idle (void *unused)
 {
 	/* endless idle loop with no priority at all */
 	while (1) {
+		void (*idle)(void) = pm_idle;
+		if (!idle)
+			idle = default_idle;
+
 #ifdef CONFIG_SMP
 		if (!need_resched())
 			min_xtp();
@@ -186,10 +193,7 @@ cpu_idle (void *unused)
 #ifdef CONFIG_IA64_SGI_SN
 			snidle();
 #endif
-			if (pm_idle)
-				(*pm_idle)();
-			else
-				default_idle();
+			(*idle)();
 		}
 
 #ifdef CONFIG_IA64_SGI_SN

include/asm-ia64/system.h

@@ -108,7 +108,7 @@ ia64_insn_group_barrier (void)
 #define set_mb(var, value)	do { (var) = (value); mb(); } while (0)
 #define set_wmb(var, value)	do { (var) = (value); mb(); } while (0)
 
-#define safe_halt()         ia64_pal_halt(1)                /* PAL_HALT */
+#define safe_halt()         ia64_pal_halt_light()    /* PAL_HALT_LIGHT */
 
 /*
  * The group barrier in front of the rsm & ssm are necessary to ensure