net: stmmac: Avoid DMA_CHAN_CONTROL write if no Split Header support

The driver assumes that split headers can be enabled/disabled without
stopping/starting the device, so it writes DMA_CHAN_CONTROL from
stmmac_set_features().  However, on my system (IP v5.10a without Split
Header support), simply writing DMA_CHAN_CONTROL when DMA is running
(for example, with the commands below) leads to a TX watchdog timeout.

 host$ socat TCP-LISTEN:1024,fork,reuseaddr - &
 device$ ethtool -K eth0 tso off
 device$ ethtool -K eth0 tso on
 device$ dd if=/dev/zero bs=1M count=10 | socat - TCP4:host:1024
 <tx watchdog timeout>

Note that since my IP is configured without Split Header support, the
driver always just reads and writes the same value to the
DMA_CHAN_CONTROL register.

I don't have access to any platforms with Split Header support so I
don't know if these writes to the DMA_CHAN_CONTROL while DMA is running
actually work properly on such systems.  I could not find anything in
the databook that says that DMA_CHAN_CONTROL should not be written when
the DMA is running.

But on systems without Split Header support, there is in any case no
need to call enable_sph() in stmmac_set_features() at all since SPH can
never be toggled, so we can avoid the watchdog timeout there by skipping
this call.

Fixes: 8c6fc097 ("net: stmmac: gmac4+: Add Split Header support")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -5540,8 +5540,6 @@ static int stmmac_set_features(struct net_device *netdev,
 			       netdev_features_t features)
 {
 	struct stmmac_priv *priv = netdev_priv(netdev);
-	bool sph_en;
-	u32 chan;
 
 	/* Keep the COE Type in case of csum is supporting */
 	if (features & NETIF_F_RXCSUM)
@@ -5553,10 +5551,13 @@ static int stmmac_set_features(struct net_device *netdev,
 	 */
 	stmmac_rx_ipc(priv, priv->hw);
 
-	sph_en = (priv->hw->rx_csum > 0) && priv->sph;
+	if (priv->sph_cap) {
+		bool sph_en = (priv->hw->rx_csum > 0) && priv->sph;
+		u32 chan;
 
 		for (chan = 0; chan < priv->plat->rx_queues_to_use; chan++)
 			stmmac_enable_sph(priv, priv->ioaddr, sph_en, chan);
+	}
 
 	return 0;
 }