[RFC v8 16/28] clockevents: min delta increment: calculate min_delta_ns from ticks

[Nicolai Stange <nicstange@gmail.com>]

The use of a clockevent device's ->min_delta_ns in the event programming
path hinders upcoming changes to the clockevent core making it NTP
correction aware: both, ->mult and ->min_delta_ns would need to get
updated as well as consumed atomically and we'd rather like to avoid any
locking here.

We already have got ->min_delta_ticks_adjusted which
- resembles the value of ->min_delta_ns
- and is guaranteed to be always >= the hardware's hard limit
  ->min_delta_ticks and thus, can be used w/o locking as we don't care
  for small deviations.

In clockevents_increase_min_delta(), don't use ->min_delta_ns but
calculate it dynamically from ->min_delta_ticks_adjusted.

As clockevents_increase_min_delta() gets invoked only rarely, the
additional division should not be an issue from a performance standpoint.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 kernel/time/clockevents.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 74b05dd..36dd024 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -209,29 +209,32 @@ int clockevents_tick_resume(struct clock_event_device *dev)
  */
 static int clockevents_increase_min_delta(struct clock_event_device *dev)
 {
+	u64 min_delta_ns = cev_delta2ns(dev->min_delta_ticks_adjusted, dev,
+					false);
+
 	/* Nothing to do if we already reached the limit */
-	if (dev->min_delta_ns >= MIN_DELTA_LIMIT) {
+	if (min_delta_ns >= MIN_DELTA_LIMIT) {
 		printk_deferred(KERN_WARNING
 				"CE: Reprogramming failure. Giving up\n");
 		dev->next_event.tv64 = KTIME_MAX;
 		return -ETIME;
 	}
 
-	if (dev->min_delta_ns < 5000)
-		dev->min_delta_ns = 5000;
+	if (min_delta_ns < 5000)
+		min_delta_ns = 5000;
 	else
-		dev->min_delta_ns += dev->min_delta_ns >> 1;
+		min_delta_ns += min_delta_ns >> 1;
 
-	if (dev->min_delta_ns > MIN_DELTA_LIMIT)
-		dev->min_delta_ns = MIN_DELTA_LIMIT;
+	if (min_delta_ns > MIN_DELTA_LIMIT)
+		min_delta_ns = MIN_DELTA_LIMIT;
 
-	dev->min_delta_ticks_adjusted = (unsigned long)((dev->min_delta_ns *
+	dev->min_delta_ticks_adjusted = (unsigned long)((min_delta_ns *
 						dev->mult) >> dev->shift);
 
 	printk_deferred(KERN_WARNING
 			"CE: %s increased min_delta_ns to %llu nsec\n",
 			dev->name ? dev->name : "?",
-			(unsigned long long) dev->min_delta_ns);
+			(unsigned long long) min_delta_ns);
 	return 0;
 }
 
-- 
2.10.2