[RFC PATCH v2 02/10] timekeeping: Add function to convert realtime to base clock

[lakshmi.sowjanya.d@intel.com]

From: Lakshmi Sowjanya D <lakshmi.sowjanya.d@intel.com>

Introduce an interface ktime_real_to_base_clock(), to convert realtime
to base clock.

Convert base clock to system clock using convert_base_to_cs() in
get_device_system_crosststamp().

Add a helper function to check whether the current clocksource has same
base clock.

Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Co-developed-by: Christopher S. Hall <christopher.s.hall@intel.com>
Signed-off-by: Christopher S. Hall <christopher.s.hall@intel.com>
Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@intel.com>
---
 include/linux/timekeeping.h |   6 ++
 kernel/time/timekeeping.c   | 112 +++++++++++++++++++++++++++++++++++-
 2 files changed, 116 insertions(+), 2 deletions(-)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 0f00f382bb5d..2c3bc2eac974 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -274,12 +274,18 @@ struct system_device_crosststamp {
  *		timekeeping code to verify comparability of two cycle values.
  *		The default ID, CSID_GENERIC, does not identify a specific
  *		clocksource.
+ * @nsecs:	@cycles is in nanoseconds.
  */
 struct system_counterval_t {
 	u64			cycles;
 	enum clocksource_ids	cs_id;
+	bool			nsecs;
 };
 
+extern bool ktime_real_to_base_clock(ktime_t treal,
+				     enum clocksource_ids base_id, u64 *cycles);
+extern bool timekeeping_clocksource_has_base(enum clocksource_ids id);
+
 /*
  * Get cross timestamp between system clock and device clock
  */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 0ff065c5d25b..e68ecd77ee34 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1191,6 +1191,89 @@ static bool cycle_between(u64 before, u64 test, u64 after)
 	return false;
 }
 
+static bool convert_clock(u64 *val, u32 numerator, u32 denominator)
+{
+	u64 rem, res;
+
+	if (numerator == 0 || denominator == 0)
+		return false;
+
+	res = div64_u64_rem(*val, denominator, &rem) * numerator;
+	*val = res + div_u64(rem * numerator, denominator);
+	return true;
+}
+
+static bool convert_base_to_cs(struct system_counterval_t *scv)
+{
+	struct clocksource *cs = tk_core.timekeeper.tkr_mono.clock;
+	struct clocksource_base *base = cs->base;
+
+	/* The timestamp was taken from the time keeper clock source */
+	if (cs->id == scv->cs_id)
+		return true;
+
+	/* Check whether cs_id matches the base clock */
+	if (!base || base->id != scv->cs_id)
+		return false;
+
+	/* Avoid conversion to a less precise clock */
+	if (scv->nsecs && cs->freq_khz != 0 && base->freq_khz < cs->freq_khz) {
+		if (!convert_clock(&scv->cycles, cs->freq_khz, USEC_PER_SEC))
+			return false;
+	} else {
+		if (scv->nsecs) {
+			if (!convert_clock(&scv->cycles, base->freq_khz, USEC_PER_SEC))
+				return false;
+		}
+		if (!convert_clock(&scv->cycles, base->numerator, base->denominator))
+			return false;
+	}
+	scv->cycles += base->offset;
+	return true;
+}
+
+static bool convert_cs_to_base(u64 *cycles, enum clocksource_ids base_id)
+{
+	struct clocksource *cs = tk_core.timekeeper.tkr_mono.clock;
+	struct clocksource_base *base = cs->base;
+
+	/* Check whether base_id matches the base clock */
+	if (!base || base->id != base_id)
+		return false;
+
+	*cycles -= base->offset;
+	if (!convert_clock(cycles, base->denominator, base->numerator))
+		return false;
+	return true;
+}
+
+static u64 convert_ns_to_cs(u64 delta)
+{
+	struct tk_read_base *tkr = &tk_core.timekeeper.tkr_mono;
+
+	return div_u64((delta << tkr->shift) - tkr->xtime_nsec, tkr->mult);
+}
+
+bool ktime_real_to_base_clock(ktime_t treal, enum clocksource_ids base_id, u64 *cycles)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+	unsigned int seq;
+	u64 delta;
+
+	do {
+		seq = read_seqcount_begin(&tk_core.seq);
+		delta = (u64)treal - tk->tkr_mono.base_real;
+		if (delta > tk->tkr_mono.clock->max_idle_ns)
+			return false;
+		*cycles = tk->tkr_mono.cycle_last + convert_ns_to_cs(delta);
+		if (!convert_cs_to_base(cycles, base_id))
+			return false;
+	} while (read_seqcount_retry(&tk_core.seq, seq));
+
+	return true;
+}
+EXPORT_SYMBOL_GPL(ktime_real_to_base_clock);
+
 /**
  * get_device_system_crosststamp - Synchronously capture system/device timestamp
  * @get_time_fn:	Callback to get simultaneous device time and
@@ -1236,8 +1319,7 @@ int get_device_system_crosststamp(int (*get_time_fn)
 		 * system counter value is the same as for the currently
 		 * installed timekeeper clocksource
 		 */
-		if (system_counterval.cs_id == CSID_GENERIC ||
-		    tk->tkr_mono.clock->id != system_counterval.cs_id)
+		if (!convert_base_to_cs(&system_counterval))
 			return -ENODEV;
 		cycles = system_counterval.cycles;
 
@@ -1304,6 +1386,32 @@ int get_device_system_crosststamp(int (*get_time_fn)
 }
 EXPORT_SYMBOL_GPL(get_device_system_crosststamp);
 
+/**
+ * timekeeping_clocksource_has_base - Check whether the current clocksource
+ *	has a base clock
+ * @id:		The clocksource ID to check for
+ *
+ * Note:	The return value is a snapshot which can become invalid right
+ *	after the function returns.
+ *
+ * Return:	true if the timekeeper clocksource has a base clock with @id,
+ *	false otherwise
+ */
+bool timekeeping_clocksource_has_base(enum clocksource_ids id)
+{
+	unsigned int seq;
+	bool ret;
+
+	do {
+		seq = read_seqcount_begin(&tk_core.seq);
+		ret = tk_core.timekeeper.tkr_mono.clock->base ?
+		tk_core.timekeeper.tkr_mono.clock->base->id == id : false;
+	} while (read_seqcount_retry(&tk_core.seq, seq));
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(timekeeping_clocksource_has_base);
+
 /**
  * do_settimeofday64 - Sets the time of day.
  * @ts:     pointer to the timespec64 variable containing the new time
-- 
2.35.3