linux-kernel - [RFC v5 17/23] clockevents: use ->min_delta_ticks

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Date:   Sun,  4 Sep 2016 03:28:46 +0200
From:   Nicolai Stange <nicstange@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
        Nicolai Stange <nicstange@...il.com>
Subject: [RFC v5 17/23] clockevents: use ->min_delta_ticks_adjusted to program minimum delta

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 both implementations of clockevents_program_min_delta(),
don't calculate the event's deadline from ->min_delta_ns, but use its
drop-in replacement ->min_delta_ticks_adjusted.

Signed-off-by: Nicolai Stange <nicstange@...il.com>
---
 kernel/time/clockevents.c | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 8983fee..aa7b325 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -246,19 +246,14 @@ static int clockevents_increase_min_delta(struct clock_event_device *dev)
  */
 static int clockevents_program_min_delta(struct clock_event_device *dev)
 {
-	unsigned long long clc;
-	int64_t delta;
 	int i;
 
 	for (i = 0;;) {
-		delta = dev->min_delta_ns;
-
 		if (clockevent_state_shutdown(dev))
 			return 0;
 
 		dev->retries++;
-		clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
-		if (dev->set_next_event((unsigned long) clc, dev) == 0)
+		if (!dev->set_next_event(dev->min_delta_ticks_adjusted, dev))
 			return 0;
 
 		if (++i > 2) {
@@ -284,17 +279,11 @@ static int clockevents_program_min_delta(struct clock_event_device *dev)
  */
 static int clockevents_program_min_delta(struct clock_event_device *dev)
 {
-	unsigned long long clc;
-	int64_t delta;
-
-	delta = dev->min_delta_ns;
-
 	if (clockevent_state_shutdown(dev))
 		return 0;
 
 	dev->retries++;
-	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
-	return dev->set_next_event((unsigned long) clc, dev);
+	return dev->set_next_event(dev->min_delta_ticks_adjusted, dev);
 }
 
 #endif /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */
-- 
2.9.3