[PATCH 1/3] clockevents: Introduce CLOCK_EVT_STATE_ONESHOT_STOPPED state

[Viresh Kumar]

When no timers/hrtimers are pending, the expiry time is set to a special value:
'KTIME_MAX'. This normally happens with NO_HZ_{IDLE|FULL} in both LOWRES/HIGHRES
modes.

When 'expiry == KTIME_MAX', we either cancel the 'tick-sched' hrtimer
(NOHZ_MODE_HIGHRES) or skip reprogramming clockevent device (NOHZ_MODE_LOWRES).
But, the clockevent device is already reprogrammed from the tick-handler for
next tick.

As the clock event device is programmed in ONESHOT mode it will at least fire
one more time (unnecessarily). Timers on few implementations (like
arm_arch_timer, etc.) only support PERIODIC mode and their drivers emulate
ONESHOT over that. Which means that on these platforms we will get spurious
interrupts periodically (at last programmed interval rate, normally tick rate).

In order to avoid spurious interrupts, the clockevent device should be stopped
or its interrupts should be masked.

A simple (yet hacky) solution to get this fixed could be: update
hrtimer_force_reprogram() to always reprogram clockevent device and update
clockevent drivers to STOP generating events (or delay it to max time) when
'expires' is set to KTIME_MAX. But the drawback here is that every clockevent
driver has to be hacked for this particular case and its very easy for new ones
to miss this.

However, Thomas suggested to add an optional state ONESHOT_STOPPED to solve this
problem: lkml.org/lkml/2014/5/9/508.

This patch adds support for ONESHOT_STOPPED state in clockevents core. It will
only be available to drivers that implement the state-specific callbacks instead
of the legacy ->set_mode() callback.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 include/linux/clockchips.h |  7 ++++++-
 kernel/time/clockevents.c  | 14 +++++++++++++-
 kernel/time/timer_list.c   |  6 ++++++
 3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index e20232c3320a..33ad247f8662 100644
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -51,12 +51,15 @@ enum clock_event_mode {
  *		reached from DETACHED or SHUTDOWN.
  * ONESHOT:	Device is programmed to generate event only once. Can be reached
  *		from DETACHED or SHUTDOWN.
+ * ONESHOT_STOPPED: Device was programmed in ONESHOT mode and is temporarily
+ *		    stopped.
  */
 enum clock_event_state {
 	CLOCK_EVT_STATE_DETACHED = 0,
 	CLOCK_EVT_STATE_SHUTDOWN,
 	CLOCK_EVT_STATE_PERIODIC,
 	CLOCK_EVT_STATE_ONESHOT,
+	CLOCK_EVT_STATE_ONESHOT_STOPPED,
 };
 
 /*
@@ -103,6 +106,7 @@ enum clock_event_state {
  * @set_mode:		legacy set mode function, only for modes <= CLOCK_EVT_MODE_RESUME.
  * @set_state_periodic:	switch state to periodic, if !set_mode
  * @set_state_oneshot:	switch state to oneshot, if !set_mode
+ * @set_state_oneshot_stopped: switch state to oneshot_stopped, if !set_mode
  * @set_state_shutdown:	switch state to shutdown, if !set_mode
  * @tick_resume:	resume clkevt device, if !set_mode
  * @broadcast:		function to broadcast events
@@ -136,12 +140,13 @@ struct clock_event_device {
 	 * State transition callback(s): Only one of the two groups should be
 	 * defined:
 	 * - set_mode(), only for modes <= CLOCK_EVT_MODE_RESUME.
-	 * - set_state_{shutdown|periodic|oneshot}(), tick_resume().
+	 * - set_state_{shutdown|periodic|oneshot|oneshot_stopped}(), tick_resume().
 	 */
 	void			(*set_mode)(enum clock_event_mode mode,
 					    struct clock_event_device *);
 	int			(*set_state_periodic)(struct clock_event_device *);
 	int			(*set_state_oneshot)(struct clock_event_device *);
+	int			(*set_state_oneshot_stopped)(struct clock_event_device *);
 	int			(*set_state_shutdown)(struct clock_event_device *);
 	int			(*tick_resume)(struct clock_event_device *);
 
diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 73689df1e4b8..04f6c3433f8e 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -138,6 +138,17 @@ static int __clockevents_set_state(struct clock_event_device *dev,
 			return -ENOSYS;
 		return dev->set_state_oneshot(dev);
 
+	case CLOCK_EVT_STATE_ONESHOT_STOPPED:
+		/* Core internal bug */
+		if (WARN_ONCE(dev->state != CLOCK_EVT_STATE_ONESHOT,
+			      "Current state: %d\n", dev->state))
+			return -EINVAL;
+
+		if (dev->set_state_oneshot_stopped)
+			return dev->set_state_oneshot_stopped(dev);
+		else
+			return -ENOSYS;
+
 	default:
 		return -ENOSYS;
 	}
@@ -449,7 +460,8 @@ static int clockevents_sanity_check(struct clock_event_device *dev)
 	if (dev->set_mode) {
 		/* We shouldn't be supporting new modes now */
 		WARN_ON(dev->set_state_periodic || dev->set_state_oneshot ||
-			dev->set_state_shutdown || dev->tick_resume);
+			dev->set_state_shutdown || dev->tick_resume ||
+			dev->set_state_oneshot_stopped);
 
 		BUG_ON(dev->mode != CLOCK_EVT_MODE_UNUSED);
 		return 0;
diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 05aa5590106a..98d77969ba85 100644
--- a/kernel/time/timer_list.c
+++ b/kernel/time/timer_list.c
@@ -251,6 +251,12 @@ print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
 			SEQ_printf(m, "\n");
 		}
 
+		if (dev->set_state_oneshot_stopped) {
+			SEQ_printf(m, " oneshot stopped: ");
+			print_name_offset(m, dev->set_state_oneshot_stopped);
+			SEQ_printf(m, "\n");
+		}
+
 		if (dev->tick_resume) {
 			SEQ_printf(m, " resume:   ");
 			print_name_offset(m, dev->tick_resume);
-- 
2.3.0.rc0.44.ga94655d

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