docs: net: ethtool: document ops-locked drivers and op_needs_rtnl

Catch up various bits of documentation after the locking changes.

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20260605002912.3456868-13-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/netdev-features.rst

@@ -76,6 +76,13 @@ netdev instance lock, that lock must be held as well. This should not be
 done from ndo_*_features callbacks. netdev->features should not be modified
 by driver except by means of ndo_fix_features callback.
 
+For "ops locked" drivers (see Documentation/networking/netdevices.rst),
+ethtool callbacks that may end up invoking netdev_update_features() must
+opt back into rtnl_lock by setting the matching ETHTOOL_OP_NEEDS_RTNL_*
+bit in ``ethtool_ops::op_needs_rtnl``. The ethtool core then keeps
+rtnl_lock held across those SET callbacks so the contract above still
+holds.
+
 ndo_features_check is called for each skb before that skb is passed to
 ndo_start_xmit. Driver may perform any non-trivial checks (e.g. exact
 header geometry / length) and withdraw features like HW_CSUM or TSO,

Documentation/networking/netdevices.rst

@@ -351,10 +351,6 @@ virtual and the physical device. To prevent deadlocks, the virtual device's
 lock must always be acquired before the physical device's (see
 ``netdev_nl_queue_create_doit``).
 
-In the future, there will be an option for individual
-drivers to opt out of using ``rtnl_lock`` and instead perform their control
-operations directly under the netdev instance lock.
-
 Device drivers are encouraged to rely on the instance lock where possible.
 
 For the (mostly software) drivers that need to interact with the core stack,
@@ -375,9 +371,16 @@ the instance lock.
 struct ethtool_ops
 ------------------
 
-Similarly to ``ndos`` the instance lock is only held for select drivers.
-For "ops locked" drivers all ethtool ops without exceptions should
-be called under the instance lock.
+For non-"ops locked" drivers ethtool_ops are executed under ``rtnl_lock``.
+
+For "ops locked" drivers, ``ethtool_ops``, unlike ``ndos``, run under
+the instance lock **only**. Drivers may request that ``rtnl_lock``
+is held around specific operations (both SET and GET) by setting
+appropriate bits in ``ethtool_ops::op_needs_rtnl`` (if the necessary
+``ETHTOOL_OP_NEEDS_RTNL_*`` bit doesn't exist, just add it).
+Commonly used core helpers which force drivers to selectively opt-in to
+``rtnl_lock`` protection include ``netdev_update_features()``,
+``netif_set_real_num_tx_queues()``, and phylink helpers.
 
 struct netdev_stat_ops
 ----------------------

include/linux/ethtool.h

@@ -1177,8 +1177,14 @@ struct kernel_ethtool_ts_info {
  * @get_mm_stats: Query the 802.3 MAC Merge layer statistics.
  *
  * All operations are optional (i.e. the function pointer may be set
- * to %NULL) and callers must take this into account.  Callers must
- * hold the RTNL lock or netdev instance lock (see @op_needs_rtnl).
+ * to %NULL) and callers must take this into account.
+ *
+ * For traditional drivers callers hold ``rtnl_lock`` across the call.
+ * For "ops locked" drivers (see Documentation/networking/netdevices.rst)
+ * callers instead hold the netdev instance lock (``netdev_lock_ops``);
+ * ``rtnl_lock`` is additionally held only for callbacks for which
+ * the driver opts in via the matching ``ETHTOOL_OP_NEEDS_RTNL_*`` bit
+ * in @op_needs_rtnl.
  *
  * See the structures used by these operations for further documentation.
  * Note that for all operations using a structure ending with a zero-