MN10300: Make sched_clock() report time since boot

Make sched_clock() report time since boot rather than time since last
timer interrupt.

Make sched_clock() expand and scale the 32-bit TSC value running at
IOCLK speed (~33MHz) to a 64-bit nanosecond counter, using cnt32_to_63()
acquired from the ARM arch and without using slow DIVU instructions
every call.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 41 insertions, 11 deletions

diff --git a/arch/mn10300/kernel/time.c b/arch/mn10300/kernel/time.c
index babb7c2ac377..e4606586f94c 100644
--- a/arch/mn10300/kernel/time.c
+++ b/arch/mn10300/kernel/time.c
@@ -1,6 +1,6 @@
 /* MN10300 Low level time management
  *
- * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
+ * Copyright (C) 2007-2008 Red Hat, Inc. All Rights Reserved.
  * Written by David Howells (dhowells@redhat.com)
  * - Derived from arch/i386/kernel/time.c
  *
@@ -16,6 +16,7 @@
 #include <linux/init.h>
 #include <linux/smp.h>
 #include <linux/profile.h>
+#include <linux/cnt32_to_63.h>
 #include <asm/irq.h>
 #include <asm/div64.h>
 #include <asm/processor.h>
@@ -40,27 +41,54 @@ static struct irqaction timer_irq = {
         .name           = "timer",
 };
 
+static unsigned long sched_clock_multiplier;
+
 /*
  * scheduler clock - returns current time in nanosec units.
  */
 unsigned long long sched_clock(void)
 {
         union {
-                unsigned long long l;
-                u32 w[2];
-        } quot;
+                unsigned long long ll;
+                unsigned l[2];
+        } tsc64, result;
+        unsigned long tsc, tmp;
+        unsigned product[3]; /* 96-bit intermediate value */
+
+        /* read the TSC value
+         */
+        tsc = 0 - get_cycles(); /* get_cycles() counts down */
 
-        quot.w[0] = mn10300_last_tsc - get_cycles();
-        quot.w[1] = 1000000000;
+        /* expand to 64-bits.
+         * - sched_clock() must be called once a minute or better or the
+         *   following will go horribly wrong - see cnt32_to_63()
+         */
+        tsc64.ll = cnt32_to_63(tsc) & 0x7fffffffffffffffULL;
 
-        asm("mulu %2,%3,%0,%1"
-            : "=r"(quot.w[1]), "=r"(quot.w[0])
-            : "0"(quot.w[1]), "1"(quot.w[0])
+        /* scale the 64-bit TSC value to a nanosecond value via a 96-bit
+         * intermediate
+         */
+        asm("mulu       %2,%0,%3,%0     \n"     /* LSW * mult ->  0:%3:%0 */
+            "mulu       %2,%1,%2,%1     \n"     /* MSW * mult -> %2:%1:0 */
+            "add        %3,%1           \n"
+            "addc       0,%2            \n"     /* result in %2:%1:%0 */
+            : "=r"(product[0]), "=r"(product[1]), "=r"(product[2]), "=r"(tmp)
+            :  "0"(tsc64.l[0]),  "1"(tsc64.l[1]),  "2"(sched_clock_multiplier)
             : "cc");
 
-        do_div(quot.l, MN10300_TSCCLK);
+        result.l[0] = product[1] << 16 | product[0] >> 16;
+        result.l[1] = product[2] << 16 | product[1] >> 16;
 
-        return quot.l;
+        return result.ll;
+}
+
+/*
+ * initialise the scheduler clock
+ */
+static void __init mn10300_sched_clock_init(void)
+{
+        sched_clock_multiplier =
+                __muldiv64u(NSEC_PER_SEC, 1 << 16, MN10300_TSCCLK);
 }
 
 /*
@@ -128,4 +156,6 @@ void __init time_init(void)
         /* start the watchdog timer */
         watchdog_go();
 #endif
+
+        mn10300_sched_clock_init();
 }