timer: add API to query ticks until the next timer

It is useful to know when the next timer will expire when
using rte_epoll_wait (or sleep when idle). This experimental
API provides a hook to query the number of ticks remaining.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>

lib/librte_timer/rte_timer.c

@@ -978,6 +978,33 @@ rte_timer_stop_all(uint32_t timer_data_id, unsigned int *walk_lcores,
 	return 0;
 }
 
+int64_t
+rte_timer_next_ticks(void)
+{
+	unsigned int lcore_id = rte_lcore_id();
+	struct rte_timer_data *timer_data;
+	struct priv_timer *priv_timer;
+	const struct rte_timer *tm;
+	uint64_t cur_time;
+	int64_t left = -ENOENT;
+
+	TIMER_DATA_VALID_GET_OR_ERR_RET(default_data_id, timer_data, -EINVAL);
+
+	priv_timer = timer_data->priv_timer;
+	cur_time = rte_get_timer_cycles();
+
+	rte_spinlock_lock(&priv_timer[lcore_id].list_lock);
+	tm = priv_timer[lcore_id].pending_head.sl_next[0];
+	if (tm) {
+		left = tm->expire - cur_time;
+		if (left < 0)
+			left = 0;
+	}
+	rte_spinlock_unlock(&priv_timer[lcore_id].list_lock);
+
+	return left;
+}
+
 /* dump statistics about timers */
 static void
 __rte_timer_dump_stats(struct rte_timer_data *timer_data __rte_unused, FILE *f)

lib/librte_timer/rte_timer.h

@@ -331,6 +331,22 @@ void rte_timer_stop_sync(struct rte_timer *tim);
  */
 int rte_timer_pending(struct rte_timer *tim);
 
+/**
+ * @warning
+ * @b EXPERIMENTAL: this API may change without prior notice
+ *
+ * Time until the next timer on the current lcore
+ * This function gives the ticks until the next timer will be active.
+ *
+ * @return
+ *   - -EINVAL: invalid timer data instance identifier
+ *   - -ENOENT: no timer pending
+ *   - 0: a timer is pending and will run at next rte_timer_manage()
+ *   - >0: ticks until the next timer is ready
+ */
+__rte_experimental
+int64_t rte_timer_next_ticks(void);
+
 /**
  * Manage the timer list and execute callback functions.
  *

lib/librte_timer/rte_timer_version.map

@@ -23,6 +23,7 @@ EXPERIMENTAL {
 	rte_timer_alt_stop;
 	rte_timer_data_alloc;
 	rte_timer_data_dealloc;
+	rte_timer_next_ticks;
 	rte_timer_stop_all;
 	rte_timer_subsystem_finalize;
 };