[PATCH net 1/5] net: dsa: mt7530: fix FDB entries not aging out with short timeout

[Daniel Golle <daniel@makrotopia.org>]

When setting a low ageing time such as 10 seconds, the algorithm in
mt7530_set_ageing_time() finds AGE_CNT=0 and AGE_UNIT=9 as the first
exact match (starting the search from tmp_age_count=0).

On the MT7530/MT7531 hardware, the per-entry aging counter is
initialized to AGE_CNT when a MAC address is learned. With AGE_CNT=0,
new entries start with a counter value of 0, which the hardware treats
as "already aged" and never removes, effectively disabling aging.

Fix this by starting the search from tmp_age_count=1 to ensure entries
always have a non-zero initial aging counter. For a 10-second ageing
time this yields AGE_CNT=1 and AGE_UNIT=4 instead: the timer ticks
every 5 seconds and entries are removed after 2 ticks.

Fixes: ea6d5c924e39 ("net: dsa: mt7530: support setting ageing time")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
---
 drivers/net/dsa/mt7530.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index 44d670904ad8..b1903da7d500 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -1027,8 +1027,12 @@ mt7530_set_ageing_time(struct dsa_switch *ds, unsigned int msecs)
 	if (secs < 1 || secs > (AGE_CNT_MAX + 1) * (AGE_UNIT_MAX + 1))
 		return -ERANGE;
 
-	/* iterate through all possible age_count to find the closest pair */
-	for (tmp_age_count = 0; tmp_age_count <= AGE_CNT_MAX; ++tmp_age_count) {
+	/* Iterate through all possible age_count values to find the closest
+	 * pair. Start from 1 because the per-entry aging counter is
+	 * initialized to AGE_CNT and a value of 0 means the entry will
+	 * never be aged out.
+	 */
+	for (tmp_age_count = 1; tmp_age_count <= AGE_CNT_MAX; ++tmp_age_count) {
 		unsigned int tmp_age_unit = secs / (tmp_age_count + 1) - 1;
 
 		if (tmp_age_unit <= AGE_UNIT_MAX) {
-- 
2.54.0