[ARM] 3980/1: extend the ARM Versatile sched_clock implementation from 32 to 63 bit

 period

This provides a 63 bit clock counter guaranteed to be monotonic over a
period of 35583 days instead of a clock wrap every 179 seconds, as long
as sched_clock() is called at least once every 89 seconds.  This should
not be a problem in practice, although a kernel timer could be scheduled
every 80 seconds for example simply to call sched_clock() making sure
top bits are always synchronized if need be.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 10 insertions, 4 deletions

diff --git a/arch/arm/mach-versatile/core.c b/arch/arm/mach-versatile/core.c
index 3b8576111c16..998b398df30e 100644
--- a/arch/arm/mach-versatile/core.c
+++ b/arch/arm/mach-versatile/core.c
@@ -27,6 +27,7 @@
 #include <linux/amba/bus.h>
 #include <linux/amba/clcd.h>
 
+#include <asm/cnt32_to_63.h>
 #include <asm/system.h>
 #include <asm/hardware.h>
 #include <asm/io.h>
@@ -228,14 +229,19 @@ void __init versatile_map_io(void)
 
 /*
  * This is the Versatile sched_clock implementation.  This has
- * a resolution of 41.7ns, and a maximum value of about 179s.
+ * a resolution of 41.7ns, and a maximum value of about 35583 days.
+ *
+ * The return value is guaranteed to be monotonic in that range as
+ * long as there is always less than 89 seconds between successive
+ * calls to this function.
  */
 unsigned long long sched_clock(void)
 {
-        unsigned long long v;
+        unsigned long long v = cnt32_to_63(readl(VERSATILE_REFCOUNTER));
 
-        v = (unsigned long long)readl(VERSATILE_REFCOUNTER) * 125;
-        do_div(v, 3);
+        /* the <<1 gets rid of the cnt_32_to_63 top bit saving on a bic insn */
+        v *= 125<<1;
+        do_div(v, 3<<1);
 
         return v;
 }