[RFC PATCH v3 3/6] timekeeping: Expose API allowing retrival of current clocksource and counter value

[Jianyong Wu]

>From Marc Zyngier <maz@xxxxxxxxxx>
A number of PTP drivers (such as ptp-kvm) are assuming what the
current clock source is, which could lead to interesting effects on
systems where the clocksource can change depending on external events.

For this purpose, add a new API that retrives both the current
monotonic clock as well as its counter value.

>From Jianyong Wu: export this API then modules can use it.

Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>
---
 include/linux/timekeeping.h |  3 +++
 kernel/time/timekeeping.c   | 13 +++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index a8ab0f143ac4..a5389adaa8bc 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -247,6 +247,9 @@ extern int get_device_system_crosststamp(
 			struct system_time_snapshot *history,
 			struct system_device_crosststamp *xtstamp);
 
+/* Obtain current monotonic clock and its counter value  */
+extern void get_current_counterval(struct system_counterval_t *sc);
+
 /*
  * Simultaneously snapshot realtime and monotonic raw clocks
  */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 44b726bab4bd..07a0969625b1 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1098,6 +1098,19 @@ static bool cycle_between(u64 before, u64 test, u64 after)
 	return false;
 }
 
+/**
+ * get_current_counterval - Snapshot the current clocksource and counter value
+ * @sc:	Pointer to a struct containing the current clocksource and its value
+ */
+void get_current_counterval(struct system_counterval_t *sc)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+
+	sc->cs = READ_ONCE(tk->tkr_mono.clock);
+	sc->cycles = sc->cs->read(sc->cs);
+}
+EXPORT_SYMBOL_GPL(get_current_counterval);
+
 /**
  * get_device_system_crosststamp - Synchronously capture system/device timestamp
  * @get_time_fn:	Callback to get simultaneous device time and
-- 
2.17.1