net: ethernet: ravb: exit if re-initialization fails in tx timeout

According to the report of [1], this driver is possible to cause
the following error in ravb_tx_timeout_work().

ravb e6800000.ethernet ethernet: failed to switch device to config mode

This error means that the hardware could not change the state
from "Operation" to "Configuration" while some tx and/or rx queue
are operating. After that, ravb_config() in ravb_dmac_init() will fail,
and then any descriptors will be not allocaled anymore so that NULL
pointer dereference happens after that on ravb_start_xmit().

To fix the issue, the ravb_tx_timeout_work() should check
the return values of ravb_stop_dma() and ravb_dmac_init().
If ravb_stop_dma() fails, ravb_tx_timeout_work() re-enables TX and RX
and just exits. If ravb_dmac_init() fails, just exits.

[1]
https://lore.kernel.org/linux-renesas-soc/20200518045452.2390-1-dirk.behme@de.bosch.com/Reported-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/ravb_main.c

@@ -1450,6 +1450,7 @@ static void ravb_tx_timeout_work(struct work_struct *work)
 	struct ravb_private *priv = container_of(work, struct ravb_private,
 						 work);
 	struct net_device *ndev = priv->ndev;
+	int error;
 
 	netif_tx_stop_all_queues(ndev);
 
@@ -1458,15 +1459,36 @@ static void ravb_tx_timeout_work(struct work_struct *work)
 		ravb_ptp_stop(ndev);
 
 	/* Wait for DMA stopping */
-	ravb_stop_dma(ndev);
+	if (ravb_stop_dma(ndev)) {
+		/* If ravb_stop_dma() fails, the hardware is still operating
+		 * for TX and/or RX. So, this should not call the following
+		 * functions because ravb_dmac_init() is possible to fail too.
+		 * Also, this should not retry ravb_stop_dma() again and again
+		 * here because it's possible to wait forever. So, this just
+		 * re-enables the TX and RX and skip the following
+		 * re-initialization procedure.
+		 */
+		ravb_rcv_snd_enable(ndev);
+		goto out;
+	}
 
 	ravb_ring_free(ndev, RAVB_BE);
 	ravb_ring_free(ndev, RAVB_NC);
 
 	/* Device init */
-	ravb_dmac_init(ndev);
+	error = ravb_dmac_init(ndev);
+	if (error) {
+		/* If ravb_dmac_init() fails, descriptors are freed. So, this
+		 * should return here to avoid re-enabling the TX and RX in
+		 * ravb_emac_init().
+		 */
+		netdev_err(ndev, "%s: ravb_dmac_init() failed, error %d\n",
+			   __func__, error);
+		return;
+	}
 	ravb_emac_init(ndev);
 
+out:
 	/* Initialise PTP Clock driver */
 	if (priv->chip_id == RCAR_GEN2)
 		ravb_ptp_init(ndev, priv->pdev);