time: Add timekeeping snapshot code capturing system time and counter

In the current timekeeping code there isn't any interface to
atomically capture the current relationship between the system counter
and system time. ktime_get_snapshot() returns this triple (counter,
monotonic raw, realtime) in the system_time_snapshot struct.

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: kevin.b.stanton@intel.com
Cc: kevin.j.clarke@intel.com
Cc: hpa@zytor.com
Cc: jeffrey.t.kirsher@intel.com
Cc: netdev@vger.kernel.org
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Christopher S. Hall <christopher.s.hall@intel.com>
[jstultz: Moved structure definitions around to clean things up,
 fixed cycles_t/cycle_t confusion.]
Signed-off-by: John Stultz <john.stultz@linaro.org>

2 files changed, 48 insertions, 0 deletions

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index ec89d846324c..7817591af46f 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -267,6 +267,24 @@ extern void ktime_get_raw_and_real_ts64(struct timespec64 *ts_raw,
                                         struct timespec64 *ts_real);
 
 /*
+ * struct system_time_snapshot - simultaneous raw/real time capture with
+ *      counter value
+ * @cycles:     Clocksource counter value to produce the system times
+ * @real:       Realtime system time
+ * @raw:        Monotonic raw system time
+ */
+struct system_time_snapshot {
+        cycle_t         cycles;
+        ktime_t         real;
+        ktime_t         raw;
+};
+
+/*
+ * Simultaneously snapshot realtime and monotonic raw clocks
+ */
+extern void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot);
+
+/*
  * Persistent clock related interfaces
  */
 extern int persistent_clock_is_local;
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 4243d28177ac..89b4695bd083 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -874,6 +874,36 @@ time64_t __ktime_get_real_seconds(void)
         return tk->xtime_sec;
 }
 
+/**
+ * ktime_get_snapshot - snapshots the realtime/monotonic raw clocks with counter
+ * @systime_snapshot:   pointer to struct receiving the system time snapshot
+ */
+void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot)
+{
+        struct timekeeper *tk = &tk_core.timekeeper;
+        unsigned long seq;
+        ktime_t base_raw;
+        ktime_t base_real;
+        s64 nsec_raw;
+        s64 nsec_real;
+        cycle_t now;
+
+        do {
+                seq = read_seqcount_begin(&tk_core.seq);
+
+                now = tk->tkr_mono.read(tk->tkr_mono.clock);
+                base_real = ktime_add(tk->tkr_mono.base,
+                                      tk_core.timekeeper.offs_real);
+                base_raw = tk->tkr_raw.base;
+                nsec_real = timekeeping_cycles_to_ns(&tk->tkr_mono, now);
+                nsec_raw  = timekeeping_cycles_to_ns(&tk->tkr_raw, now);
+        } while (read_seqcount_retry(&tk_core.seq, seq));
+
+        systime_snapshot->cycles = now;
+        systime_snapshot->real = ktime_add_ns(base_real, nsec_real);
+        systime_snapshot->raw = ktime_add_ns(base_raw, nsec_raw);
+}
+EXPORT_SYMBOL_GPL(ktime_get_snapshot);
 
 #ifdef CONFIG_NTP_PPS