Mask measure blips end to end and show a running median cable length

bcm.go

@@ -45,8 +45,9 @@ const (
 	// Covers the handler's documented 2 s freeze during 10GBASE-T training.
 	bcmStatusTimeout = 3 * time.Second
 
-	ecdPoll     = 200 * time.Millisecond
-	ecdDeadline = 50 * time.Second
+	ecdPoll        = 200 * time.Millisecond
+	ecdArmDeadline = 2 * time.Second
+	ecdDeadline    = 50 * time.Second
 )
 
 type bcm struct {
@@ -272,6 +273,24 @@ func (b *bcm) cableDiag() (res ecdResult, err error) {
 		if err = b.mdioWrite(bcmMMDVendor, bcmRegECDCtrl, ctrl&^0xF400|0x8400); err != nil {
 			return
 		}
+		// A diag takes seconds against a 200 ms poll, so busy must be seen set
+		// before clear means finished; accepting an early clear reads the
+		// result of a diag that never armed, which is an instant all-zero
+		// "success" while restartAN still tears the link down.
+		arm := time.Now().Add(ecdArmDeadline)
+		for {
+			if ctrl, err = b.mdioRead(bcmMMDVendor, bcmRegECDCtrl); err != nil {
+				return
+			}
+			if ctrl&0x0800 != 0 {
+				break
+			}
+			if time.Now().After(arm) {
+				err = fmt.Errorf("%s: cable diag never armed within %s", b.ifname, ecdArmDeadline)
+				return
+			}
+			time.Sleep(ecdPoll)
+		}
 		deadline := time.Now().Add(ecdDeadline)
 		for {
 			if ctrl, err = b.mdioRead(bcmMMDVendor, bcmRegECDCtrl); err != nil {