[PATCH 2/9] timer: Add comments about calc_index() ceiling work

[Frederic Weisbecker <frederic@kernel.org>]

calc_index() adds 1 unit of the level granularity to the expiry passed
in parameter to ensure that the timer doesn't expire too early. Add a
comment to explain that and the resulting layout in the wheel.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4c977df3610b..ae1259f67486 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -156,7 +156,8 @@ EXPORT_SYMBOL(jiffies_64);
 
 /*
  * The time start value for each level to select the bucket at enqueue
- * time.
+ * time. We start from the last possible delta of the previous level
+ * so that we can later add an extra LVL_GRAN(n) to n (see calc_index()).
  */
 #define LVL_START(n)	((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT))
 
@@ -489,6 +490,13 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx)
  */
 static inline unsigned calc_index(unsigned expires, unsigned lvl)
 {
+	/*
+	 * Time may have past since the clock last reached an index of
+	 * this @lvl. And that time, below LVL_GRAN(@lvl), is going to
+	 * be substracted from the delta until we reach @expires. To
+	 * fix that we must add one level granularity unit to make sure
+	 * we rather expire late than early. Prefer ceil over floor.
+	 */
 	expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
 	return LVL_OFFS(lvl) + (expires & LVL_MASK);
 }
-- 
2.26.2