[RFC v6 17/23] clockevents: use ->min_delta_ticks_adjusted to program minimum delta

Nicolai Stange
2016-09-09T20:19:57Z

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@gmail.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