alpha: Primitive support for CPU power down.

Use WTINT to wait for the next interrupt.  Squash the WTINT call
if the PALcode doesn't support it (e.g. MILO).  No attempt is yet
made to skip clock ticks during normal scheduling in order to stay
in power down mode longer.
Signed-off-by: Richard Henderson <rth@twiddle.net>

arch/alpha/Kconfig

@@ -586,6 +586,30 @@ config NUMA
 	  Access).  This option is for configuring high-end multiprocessor
 	  server machines.  If in doubt, say N.
 
+config ALPHA_WTINT
+	bool "Use WTINT" if ALPHA_SRM || ALPHA_GENERIC
+	default y if ALPHA_QEMU
+	default n if ALPHA_EV5 || ALPHA_EV56 || (ALPHA_EV4 && !ALPHA_LCA)
+	default n if !ALPHA_SRM && !ALPHA_GENERIC
+	default y if SMP
+	---help---
+	  The Wait for Interrupt (WTINT) PALcall attempts to place the CPU
+	  to sleep until the next interrupt.  This may reduce the power
+	  consumed, and the heat produced by the computer.  However, it has
+	  the side effect of making the cycle counter unreliable as a timing
+	  device across the sleep.
+
+	  For emulation under QEMU, definitely say Y here, as we have other
+	  mechanisms for measuring time than the cycle counter.
+
+	  For EV4 (but not LCA), EV5 and EV56 systems, or for systems running
+	  MILO, sleep mode is not supported so you might as well say N here.
+
+	  For SMP systems we cannot use the cycle counter for timing anyway,
+	  so you might as well say Y here.
+
+	  If unsure, say N.
+
 config NODES_SHIFT
 	int
 	default "7"

arch/alpha/include/asm/pal.h

@@ -89,6 +89,7 @@ __CALL_PAL_W1(wrmces, unsigned long);
 __CALL_PAL_RW2(wrperfmon, unsigned long, unsigned long, unsigned long);
 __CALL_PAL_W1(wrusp, unsigned long);
 __CALL_PAL_W1(wrvptptr, unsigned long);
+__CALL_PAL_RW1(wtint, unsigned long, unsigned long);
 
 /*
  * TB routines..

arch/alpha/include/uapi/asm/pal.h

@@ -46,6 +46,7 @@
 #define PAL_rdusp	58
 #define PAL_whami	60
 #define PAL_retsys	61
+#define PAL_wtint	62
 #define PAL_rti		63
 
 

arch/alpha/kernel/process.c

@@ -46,6 +46,23 @@
 void (*pm_power_off)(void) = machine_power_off;
 EXPORT_SYMBOL(pm_power_off);
 
+#ifdef CONFIG_ALPHA_WTINT
+/*
+ * Sleep the CPU.
+ * EV6, LCA45 and QEMU know how to power down, skipping N timer interrupts.
+ */
+void arch_cpu_idle(void)
+{
+	wtint(0);
+	local_irq_enable();
+}
+
+void arch_cpu_idle_dead(void)
+{
+	wtint(INT_MAX);
+}
+#endif /* ALPHA_WTINT */
+
 struct halt_info {
 	int mode;
 	char *restart_cmd;

arch/alpha/kernel/traps.c

@@ -241,6 +241,21 @@ do_entIF(unsigned long type, struct pt_regs *regs)
 			       (const char *)(data[1] | (long)data[2] << 32), 
 			       data[0]);
 		}
+#ifdef CONFIG_ALPHA_WTINT
+		if (type == 4) {
+			/* If CALL_PAL WTINT is totally unsupported by the
+			   PALcode, e.g. MILO, "emulate" it by overwriting
+			   the insn.  */
+			unsigned int *pinsn
+			  = (unsigned int *) regs->pc - 1;
+			if (*pinsn == PAL_wtint) {
+				*pinsn = 0x47e01400; /* mov 0,$0 */
+				imb();
+				regs->r0 = 0;
+				return;
+			}
+		}
+#endif /* ALPHA_WTINT */
 		die_if_kernel((type == 1 ? "Kernel Bug" : "Instruction fault"),
 			      regs, type, NULL);
 	}