[PATCH net v2 1/5] net: dsa: mt7530: fix FDB entries not aging out with short timeout

[Daniel Golle <daniel@makrotopia.org>]

The DSA forwarding selftests bridge_vlan_aware.sh and
bridge_vlan_unaware.sh configure the bridge with ageing_time set to
LOW_AGEING_TIME (1000 centiseconds, i.e. 10 seconds) and then run
learning_test() in lib.sh, which expects a learned FDB entry to be
removed after ageing_time + 10 seconds. On MT7530/MT7531 the entry
persisted past the deadline and the "Found FDB record when should
not" assertion failed.

With msecs=10000, 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). The per-entry aging counter is
initialized to AGE_CNT when a MAC address is learned, so 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.

Starting the search at AGE_CNT=1 raises the minimum representable
ageing time from 1 to 2 seconds. Without bounds, a stale ageing_time
of 1 second would now make the loop fall through without setting
age_count and age_unit, leaving them uninitialized when written to
the MT7530_AAC hardware register. Set ds->ageing_time_min and
ds->ageing_time_max so the DSA core validates the range before the
callback is invoked, and drop the now-redundant range check from
mt7530_set_ageing_time().

Fixes: ea6d5c924e39 ("net: dsa: mt7530: support setting ageing time")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
---
v2:
 - Set ds->ageing_time_min = 2 * 1000 and
   ds->ageing_time_max = (AGE_CNT_MAX + 1) * (AGE_UNIT_MAX + 1) * 1000
   in mt7530_setup() and mt7531_setup_common() so the DSA core
   rejects out-of-range values before calling the driver.
 - Drop the now-redundant `if (secs < 1 || ...)` range check from
   mt7530_set_ageing_time(); v1 left it in place and would have
   written uninitialized age_count/age_unit to MT7530_AAC for
   secs == 1 (loop falls through without ever finding a match
   once tmp_age_count starts at 1).
 - Reworked commit message to lead with the bridge_vlan_aware.sh /
   bridge_vlan_unaware.sh learning_test() scenario that exposes
   the bug, walk through the search-loop math for LOW_AGEING_TIME
   (10 s), and document the new lower bound.

 drivers/net/dsa/mt7530.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index 44d670904ad8..cd311dfd3600 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -1023,12 +1023,16 @@ mt7530_set_ageing_time(struct dsa_switch *ds, unsigned int msecs)
 	unsigned int age_count;
 	unsigned int age_unit;
 
-	/* Applied timer is (AGE_CNT + 1) * (AGE_UNIT + 1) seconds */
-	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) {
+	/* Applied timer is (AGE_CNT + 1) * (AGE_UNIT + 1) seconds.
+	 * The DSA core has already validated the range using
+	 * ds->ageing_time_min and ds->ageing_time_max.
+	 *
+	 * 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) {
@@ -2428,6 +2432,8 @@ mt7530_setup(struct dsa_switch *ds)
 
 	ds->assisted_learning_on_cpu_port = true;
 	ds->mtu_enforcement_ingress = true;
+	ds->ageing_time_min = 2 * 1000;
+	ds->ageing_time_max = (AGE_CNT_MAX + 1) * (AGE_UNIT_MAX + 1) * 1000;
 
 	if (priv->id == ID_MT7530) {
 		regulator_set_voltage(priv->core_pwr, 1000000, 1000000);
@@ -2617,6 +2623,8 @@ mt7531_setup_common(struct dsa_switch *ds)
 
 	ds->assisted_learning_on_cpu_port = true;
 	ds->mtu_enforcement_ingress = true;
+	ds->ageing_time_min = 2 * 1000;
+	ds->ageing_time_max = (AGE_CNT_MAX + 1) * (AGE_UNIT_MAX + 1) * 1000;
 
 	mt753x_trap_frames(priv);
 
-- 
2.54.0