sfc: Wait for the link to stay up before running loopback selftest

It's been observed that some phys (such as the qt2025c) can
do down-up-down-up transitions, presumably as pcs block lock
settles down.

The loopback selftest will start sending data immediately
after the link comes up. Work around this by waiting for
the link state to stay up for two consecutive polls, rather
than one.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/sfc/selftest.c

@@ -545,7 +545,7 @@ efx_test_loopback(struct efx_tx_queue *tx_queue,
 static int efx_wait_for_link(struct efx_nic *efx)
 {
 	struct efx_link_state *link_state = &efx->link_state;
-	int count;
+	int count, link_up_count = 0;
 	bool link_up;
 
 	for (count = 0; count < 40; count++) {
@@ -567,8 +567,12 @@ static int efx_wait_for_link(struct efx_nic *efx)
 			link_up = !efx->mac_op->check_fault(efx);
 		mutex_unlock(&efx->mac_lock);
 
-		if (link_up)
-			return 0;
+		if (link_up) {
+			if (++link_up_count == 2)
+				return 0;
+		} else {
+			link_up_count = 0;
+		}
 	}
 
 	return -ETIMEDOUT;