Force master/slave roles at bringup (A master / B slave, IEEE 7.32): the AN role lottery plus per-training DSP convergence moves per-pair SNR up to ~3.6 dB, so sessions are pinned to identical conditions; launch power documented firmware-pinned (1.130), single-training SNR documented as a weak qualification number

phy.go

@@ -313,6 +313,24 @@ func (b *bcm) forceJumbo() error {
 	return err
 }
 
+// Left to AN, master/slave is a per-training lottery and each training's DSP
+// convergence moves per-pair SNR by up to ~3.6 dB; pinned roles at least keep
+// every session measured under identical conditions.
+func (b *bcm) forceRole(master bool) error {
+	defer b.acquire()()
+	v, err := b.mdioRead(7, 32)
+	if err != nil {
+		return err
+	}
+	v |= 0x8000
+	if master {
+		v |= 0x4000
+	} else {
+		v &^= 0x4000
+	}
+	return b.mdioWrite(7, 32, v)
+}
+
 func (b *bcm) restartAN() error {
 	defer b.acquire()()
 	v, err := b.mdioRead(7, 0)
@@ -822,6 +840,15 @@ func moduleChecks(mods []*phyModule, names [2]string) []checkResult {
 		}
 		out = append(out, res)
 
+		res = checkResult{item: names[i] + " role", state: "forced master"}
+		if i > 0 {
+			res.state = "forced slave"
+		}
+		if err := m.bcm.forceRole(i == 0); err != nil {
+			return fail(res.item, err)
+		}
+		out = append(out, res)
+
 		if err := m.bcm.restartAN(); err != nil {
 			return fail(names[i]+" retrain", err)
 		}