In case the time arguments used for flexible PPS signal generation are in
the past, consider the arguments to be a time offset relative to the MAC
system time.

This way, past time use case is handled and it avoids the tedious work
of passing an absolute time value for the flexible PPS signal generation
while not breaking existing scripts that may rely on this behavior.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c | 31 ++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
index 3767ba495e78d210b0529ee1754e5331f2dd0a47..5c712b33851081b5ae1dbf2a0988919ae647a9e2 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
@@ -10,6 +10,8 @@
 #include "stmmac.h"
 #include "stmmac_ptp.h"
 
+#define PTP_SAFE_TIME_OFFSET_NS	500000
+
 /**
  * stmmac_adjust_freq
  *
@@ -172,6 +174,10 @@ static int stmmac_enable(struct ptp_clock_info *ptp,
 
 	switch (rq->type) {
 	case PTP_CLK_REQ_PEROUT:
+		struct timespec64 curr_time;
+		u64 target_ns = 0;
+		u64 ns = 0;
+
 		/* Reject requests with unsupported flags */
 		if (rq->perout.flags)
 			return -EOPNOTSUPP;
@@ -180,6 +186,31 @@ static int stmmac_enable(struct ptp_clock_info *ptp,
 
 		cfg->start.tv_sec = rq->perout.start.sec;
 		cfg->start.tv_nsec = rq->perout.start.nsec;
+
+		/* A time set in the past won't trigger the start of the flexible PPS generation for
+		 * the GMAC5. For some reason it does for the GMAC4 but setting a time in the past
+		 * should be addressed anyway. Therefore, any value set it the past is considered as
+		 * an offset compared to the current MAC system time.
+		 * Be aware that an offset too low may not trigger flexible PPS generation
+		 * if time spent in this configuration makes the targeted time already outdated.
+		 * To address this, add a safe time offset.
+		 */
+		if (!cfg->start.tv_sec && cfg->start.tv_nsec < PTP_SAFE_TIME_OFFSET_NS)
+			cfg->start.tv_nsec += PTP_SAFE_TIME_OFFSET_NS;
+
+		target_ns = cfg->start.tv_nsec + ((u64)cfg->start.tv_sec * NSEC_PER_SEC);
+
+		stmmac_get_systime(priv, priv->ptpaddr, &ns);
+		if (ns > TIME64_MAX - PTP_SAFE_TIME_OFFSET_NS)
+			return -EINVAL;
+
+		curr_time = ns_to_timespec64(ns);
+		if (target_ns < ns + PTP_SAFE_TIME_OFFSET_NS) {
+			cfg->start = timespec64_add_safe(cfg->start, curr_time);
+			if (cfg->start.tv_sec == TIME64_MAX)
+				return -EINVAL;
+		}
+
 		cfg->period.tv_sec = rq->perout.period.sec;
 		cfg->period.tv_nsec = rq->perout.period.nsec;
 

-- 
2.25.1