1 files changed, 44 insertions, 0 deletions

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 407c0894ef37..b65b242f04dd 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -469,6 +469,47 @@ void clocksource_touch_watchdog(void)
 #ifdef CONFIG_GENERIC_TIME
 
 /**
+ * clocksource_max_deferment - Returns max time the clocksource can be deferred
+ * @cs:         Pointer to clocksource
+ *
+ */
+static u64 clocksource_max_deferment(struct clocksource *cs)
+{
+        u64 max_nsecs, max_cycles;
+
+        /*
+         * Calculate the maximum number of cycles that we can pass to the
+         * cyc2ns function without overflowing a 64-bit signed result. The
+         * maximum number of cycles is equal to ULLONG_MAX/cs->mult which
+         * is equivalent to the below.
+         * max_cycles < (2^63)/cs->mult
+         * max_cycles < 2^(log2((2^63)/cs->mult))
+         * max_cycles < 2^(log2(2^63) - log2(cs->mult))
+         * max_cycles < 2^(63 - log2(cs->mult))
+         * max_cycles < 1 << (63 - log2(cs->mult))
+         * Please note that we add 1 to the result of the log2 to account for
+         * any rounding errors, ensure the above inequality is satisfied and
+         * no overflow will occur.
+         */
+        max_cycles = 1ULL << (63 - (ilog2(cs->mult) + 1));
+
+        /*
+         * The actual maximum number of cycles we can defer the clocksource is
+         * determined by the minimum of max_cycles and cs->mask.
+         */
+        max_cycles = min_t(u64, max_cycles, (u64) cs->mask);
+        max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult, cs->shift);
+
+        /*
+         * To ensure that the clocksource does not wrap whilst we are idle,
+         * limit the time the clocksource can be deferred by 12.5%. Please
+         * note a margin of 12.5% is used because this can be computed with
+         * a shift, versus say 10% which would require division.
+         */
+        return max_nsecs - (max_nsecs >> 5);
+}
+
+/**
  * clocksource_select - Select the best clocksource available
  *
  * Private function. Must hold clocksource_mutex when called.
@@ -564,6 +605,9 @@ static void clocksource_enqueue(struct clocksource *cs)
  */
 int clocksource_register(struct clocksource *cs)
 {
+        /* calculate max idle time permitted for this clocksource */
+        cs->max_idle_ns = clocksource_max_deferment(cs);
+
         mutex_lock(&clocksource_mutex);
         clocksource_enqueue(cs);
         clocksource_select();