2 files changed, 22 insertions, 8 deletions

diff --git a/arch/mn10300/kernel/time.c b/arch/mn10300/kernel/time.c
index f860a340acc..75da468090b 100644
--- a/arch/mn10300/kernel/time.c
+++ b/arch/mn10300/kernel/time.c
@@ -40,21 +40,17 @@ unsigned long long sched_clock(void)
                 unsigned long long ll;
                 unsigned l[2];
         } tsc64, result;
-        unsigned long tsc, tmp;
+        unsigned long tmp;
         unsigned product[3]; /* 96-bit intermediate value */
 
         /* cnt32_to_63() is not safe with preemption */
         preempt_disable();
 
-        /* read the TSC value
-         */
-        tsc = get_cycles();
-
-        /* expand to 64-bits.
+        /* expand the tsc 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;
+        tsc64.ll = cnt32_to_63(get_cycles()) & 0x7fffffffffffffffULL;
 
         preempt_enable();
 
diff --git a/include/linux/cnt32_to_63.h b/include/linux/cnt32_to_63.h
index 7605fdd1eb6..e3d8bf26e5e 100644
--- a/include/linux/cnt32_to_63.h
+++ b/include/linux/cnt32_to_63.h
@@ -61,13 +61,31 @@ union cnt32_to_63 {
  *
  * 2) this code must not be preempted for a duration longer than the
  *    32-bit counter half period minus the longest period between two
- *    calls to this code.
+ *    calls to this code;
  *
  * Those requirements ensure proper update to the state bit in memory.
  * This is usually not a problem in practice, but if it is then a kernel
  * timer should be scheduled to manage for this code to be executed often
  * enough.
  *
+ * And finally:
+ *
+ * 3) the cnt_lo argument must be seen as a globally incrementing value,
+ *    meaning that it should be a direct reference to the counter data which
+ *    can be evaluated according to a specific ordering within the macro,
+ *    and not the result of a previous evaluation stored in a variable.
+ *
+ * For example, this is wrong:
+ *
+ *      u32 partial = get_hw_count();
+ *      u64 full = cnt32_to_63(partial);
+ *      return full;
+ *
+ * This is fine:
+ *
+ *      u64 full = cnt32_to_63(get_hw_count());
+ *      return full;
+ *
  * Note that the top bit (bit 63) in the returned value should be considered
  * as garbage.  It is not cleared here because callers are likely to use a
  * multiplier on the returned value which can get rid of the top bit