net: docs: update the sample code in driver.rst

The sample code talks about single-queue devices and uses locks.
Update it to something resembling more modern code.
Make sure we mention use of READ_ONCE() / WRITE_ONCE().

Change the comment which talked about consumer on the xmit side.
AFAIU xmit is the producer and completions are a consumer.
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/driver.rst

@@ -47,30 +47,43 @@ for a driver implementing scatter-gather this means:
 
 .. code-block:: c
 
+	static u32 drv_tx_avail(struct drv_ring *dr)
+	{
+		u32 used = READ_ONCE(dr->prod) - READ_ONCE(dr->cons);
+
+		return dr->tx_ring_size - (used & bp->tx_ring_mask);
+	}
+
 	static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
 					       struct net_device *dev)
 	{
 		struct drv *dp = netdev_priv(dev);
+		struct netdev_queue *txq;
+		struct drv_ring *dr;
+		int idx;
+
+		idx = skb_get_queue_mapping(skb);
+		dr = dp->tx_rings[idx];
+		txq = netdev_get_tx_queue(dev, idx);
 
-		lock_tx(dp);
 		//...
-		/* This is a hard error log it. */
-		if (TX_BUFFS_AVAIL(dp) <= (skb_shinfo(skb)->nr_frags + 1)) {
+		/* This should be a very rare race - log it. */
+		if (drv_tx_avail(dr) <= skb_shinfo(skb)->nr_frags + 1) {
 			netif_stop_queue(dev);
-			unlock_tx(dp);
-			printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
-			       dev->name);
+			netdev_warn(dev, "Tx Ring full when queue awake!\n");
 			return NETDEV_TX_BUSY;
 		}
 
 		//... queue packet to card ...
-		//... update tx consumer index ...
 
-		if (TX_BUFFS_AVAIL(dp) <= (MAX_SKB_FRAGS + 1))
-			netif_stop_queue(dev);
+		netdev_tx_sent_queue(txq, skb->len);
+
+		//... update tx producer index using WRITE_ONCE() ...
+
+		if (!netif_txq_maybe_stop(txq, drv_tx_avail(dr),
+					  MAX_SKB_FRAGS + 1, 2 * MAX_SKB_FRAGS))
+			dr->stats.stopped++;
 
-		//...
-		unlock_tx(dp);
 		//...
 		return NETDEV_TX_OK;
 	}
@@ -79,30 +92,10 @@ And then at the end of your TX reclamation event handling:
 
 .. code-block:: c
 
-	if (netif_queue_stopped(dp->dev) &&
-	    TX_BUFFS_AVAIL(dp) > (MAX_SKB_FRAGS + 1))
-		netif_wake_queue(dp->dev);
-
-For a non-scatter-gather supporting card, the three tests simply become:
-
-.. code-block:: c
-
-		/* This is a hard error log it. */
-		if (TX_BUFFS_AVAIL(dp) <= 0)
-
-and:
-
-.. code-block:: c
-
-		if (TX_BUFFS_AVAIL(dp) == 0)
-
-and:
-
-.. code-block:: c
+	//... update tx consumer index using WRITE_ONCE() ...
 
-	if (netif_queue_stopped(dp->dev) &&
-	    TX_BUFFS_AVAIL(dp) > 0)
-		netif_wake_queue(dp->dev);
+	netif_txq_completed_wake(txq, cmpl_pkts, cmpl_bytes,
+				 drv_tx_avail(dr), 2 * MAX_SKB_FRAGS);
 
 Lockless queue stop / wake helper macros
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~