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>

1 files changed, 24 insertions, 0 deletions

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index a67b971c830a..f9e84da49716 100644
--- a/arch/alpha/Kconfig
+++ b/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"