To be able to predict the sleep duration for a CPU that is entering idle,
knowing when the next timer/tick is going to expire, is extremely useful.
Both the teo and the menu cpuidle governors already makes use of this
information, while selecting an idle state.

Moving forward, the similar prediction needs to be done, but for a group of
idle CPUs rather than for a single idle CPU. Following changes implements a
new genpd governor, which needs this.

Support this, by sharing a new function called
tick_nohz_get_next_hrtimer(), which returns the next hrtimer or the next
tick, whatever that expires first.

Additionally, when cpuidle is about to invoke the ->enter() callback, then
call tick_nohz_get_next_hrtimer() and store its return value in the per CPU
struct cpuidle_device, as to make it available outside cpuidle.

Do note, at the point when cpuidle calls tick_nohz_get_next_hrtimer(), the
governor's ->select() callback has already made a decision whether to stop
the tick or not. In this way, tick_nohz_get_next_hrtimer() actually returns
the next timer expiration, whatever origin.

Cc: Lina Iyer <redacted>
Co-developed-by: Lina Iyer <redacted>
Co-developed-by: Daniel Lezcano <redacted>
Signed-off-by: Ulf Hansson <redacted>
---

Changes in v12:
	- New patch.

---
 drivers/cpuidle/cpuidle.c |  8 ++++++++
 include/linux/cpuidle.h   |  1 +
 include/linux/tick.h      |  7 ++++++-
 kernel/time/tick-sched.c  | 12 ++++++++++++
 4 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 7f108309e871..255365b1a6ab 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c

@@ -328,6 +328,14 @@ int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 int cpuidle_enter(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		  int index)
 {
+	/*
+	 * Store the next hrtimer, which becomes either next tick or the next
+	 * timer event, whatever expires first. Additionally, to make this data
+	 * useful for consumers outside cpuidle, we rely on that the governor's
+	 * ->select() callback have decided, whether to stop the tick or not.
+	 */
+	dev->next_hrtimer = tick_nohz_get_next_hrtimer();