docs: net: reformat driver.rst from a list to sections

driver.rst had a historical form of list of common problems.
In the age os Sphinx and rendered documentation it's better
to use the more usual title + text format.

This will allow us to render kdoc into the output more naturally.

No changes to the actual text.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/driver.rst

@@ -4,15 +4,19 @@
 Softnet Driver Issues
 =====================
 
-Transmit path guidelines:
+Transmit path guidelines
+========================
 
-1) The ndo_start_xmit method must not return NETDEV_TX_BUSY under
+Stop queues in advance
-   any normal circumstances.  It is considered a hard error unless
+----------------------
-   there is no way your device can tell ahead of time when its
-   transmit function will become busy.
 
-   Instead it must maintain the queue properly.  For example,
+The ndo_start_xmit method must not return NETDEV_TX_BUSY under
-   for a driver implementing scatter-gather this means::
+any normal circumstances.  It is considered a hard error unless
+there is no way your device can tell ahead of time when its
+transmit function will become busy.
+
+Instead it must maintain the queue properly.  For example,
+for a driver implementing scatter-gather this means::
 
 	static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb,
 					       struct net_device *dev)
@@ -42,56 +46,73 @@ Transmit path guidelines:
 		return NETDEV_TX_OK;
 	}
 
-   And then at the end of your TX reclamation event handling::
+And then at the end of your TX reclamation event handling::
 
 	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::
+For a non-scatter-gather supporting card, the three tests simply become::
 
 		/* This is a hard error log it. */
 		if (TX_BUFFS_AVAIL(dp) <= 0)
 
-   and::
+and::
 
 		if (TX_BUFFS_AVAIL(dp) == 0)
 
-   and::
+and::
 
 	if (netif_queue_stopped(dp->dev) &&
 	    TX_BUFFS_AVAIL(dp) > 0)
 		netif_wake_queue(dp->dev);
 
-2) An ndo_start_xmit method must not modify the shared parts of a
+No exclusive ownership
-   cloned SKB.
+----------------------
 
-3) Do not forget that once you return NETDEV_TX_OK from your
+An ndo_start_xmit method must not modify the shared parts of a
-   ndo_start_xmit method, it is your driver's responsibility to free
+cloned SKB.
-   up the SKB and in some finite amount of time.
 
-   For example, this means that it is not allowed for your TX
+Timely completions
-   mitigation scheme to let TX packets "hang out" in the TX
+------------------
-   ring unreclaimed forever if no new TX packets are sent.
-   This error can deadlock sockets waiting for send buffer room
-   to be freed up.
 
-   If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you
+Do not forget that once you return NETDEV_TX_OK from your
-   must not keep any reference to that SKB and you must not attempt
+ndo_start_xmit method, it is your driver's responsibility to free
-   to free it up.
+up the SKB and in some finite amount of time.
 
-Probing guidelines:
+For example, this means that it is not allowed for your TX
+mitigation scheme to let TX packets "hang out" in the TX
+ring unreclaimed forever if no new TX packets are sent.
+This error can deadlock sockets waiting for send buffer room
+to be freed up.
 
-1) Any hardware layer address you obtain for your device should
+If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you
-   be verified.  For example, for ethernet check it with
+must not keep any reference to that SKB and you must not attempt
-   linux/etherdevice.h:is_valid_ether_addr()
+to free it up.
 
-Close/stop guidelines:
+Probing guidelines
+==================
 
-1) After the ndo_stop routine has been called, the hardware must
+Address validation
-   not receive or transmit any data.  All in flight packets must
+------------------
-   be aborted. If necessary, poll or wait for completion of
-   any reset commands.
 
-2) The ndo_stop routine will be called by unregister_netdevice
+Any hardware layer address you obtain for your device should
-   if device is still UP.
+be verified.  For example, for ethernet check it with
+linux/etherdevice.h:is_valid_ether_addr()
+
+Close/stop guidelines
+=====================
+
+Quiescence
+----------
+
+After the ndo_stop routine has been called, the hardware must
+not receive or transmit any data.  All in flight packets must
+be aborted. If necessary, poll or wait for completion of
+any reset commands.
+
+Auto-close
+----------
+
+The ndo_stop routine will be called by unregister_netdevice
+if device is still UP.