net: axienet: allow setups without MDIO

In setups with fixed-link settings there is no mdio node in DTS.
axienet_probe() already handles that gracefully but lp->mii_bus is
then NULL.

Fix code that tries to blindly grab the MDIO lock by introducing two helper
functions that make the locking conditional.
Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/xilinx/xilinx_axienet.h

@@ -504,6 +504,18 @@ static inline u32 axinet_ior_read_mcr(struct axienet_local *lp)
 	return axienet_ior(lp, XAE_MDIO_MCR_OFFSET);
 }
 
+static inline void axienet_lock_mii(struct axienet_local *lp)
+{
+	if (lp->mii_bus)
+		mutex_lock(&lp->mii_bus->mdio_lock);
+}
+
+static inline void axienet_unlock_mii(struct axienet_local *lp)
+{
+	if (lp->mii_bus)
+		mutex_unlock(&lp->mii_bus->mdio_lock);
+}
+
 /**
  * axienet_iow - Memory mapped Axi Ethernet register write
  * @lp:         Pointer to axienet local structure

drivers/net/ethernet/xilinx/xilinx_axienet_main.c

@@ -1053,9 +1053,9 @@ static int axienet_open(struct net_device *ndev)
 	 * including the MDIO. MDIO must be disabled before resetting.
 	 * Hold MDIO bus lock to avoid MDIO accesses during the reset.
 	 */
-	mutex_lock(&lp->mii_bus->mdio_lock);
+	axienet_lock_mii(lp);
 	ret = axienet_device_reset(ndev);
-	mutex_unlock(&lp->mii_bus->mdio_lock);
+	axienet_unlock_mii(lp);
 
 	ret = phylink_of_phy_connect(lp->phylink, lp->dev->of_node, 0);
 	if (ret) {
@@ -1148,9 +1148,9 @@ static int axienet_stop(struct net_device *ndev)
 	}
 
 	/* Do a reset to ensure DMA is really stopped */
-	mutex_lock(&lp->mii_bus->mdio_lock);
+	axienet_lock_mii(lp);
 	__axienet_device_reset(lp);
-	mutex_unlock(&lp->mii_bus->mdio_lock);
+	axienet_unlock_mii(lp);
 
 	cancel_work_sync(&lp->dma_err_task);
 
@@ -1709,9 +1709,9 @@ static void axienet_dma_err_handler(struct work_struct *work)
 	 * including the MDIO. MDIO must be disabled before resetting.
 	 * Hold MDIO bus lock to avoid MDIO accesses during the reset.
 	 */
-	mutex_lock(&lp->mii_bus->mdio_lock);
+	axienet_lock_mii(lp);
 	__axienet_device_reset(lp);
-	mutex_unlock(&lp->mii_bus->mdio_lock);
+	axienet_unlock_mii(lp);
 
 	for (i = 0; i < lp->tx_bd_num; i++) {
 		cur_p = &lp->tx_bd_v[i];