phydev <> netdev linking and lifecycle depends on rtnl_lock.
We want to switch to instance locks for most ethtool ops.
Let's add an assert that ops locked devices don't use phydev
today. If one does we can either opt the phy ops out of
being purely ops locked, or do deeper surgery to make phy
locking ops-compatible. I don't think there's any fundamental
challenge to make that work.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 include/linux/phy_link_topology.h   | 5 +++++
 drivers/net/phy/phy_link_topology.c | 8 ++++++++
 net/ethtool/netlink.c               | 6 ++++--
 net/ethtool/phy.c                   | 1 -
 4 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/phy_link_topology.h b/include/linux/phy_link_topology.h
index 68a59e25821c..95575f68d5bc 100644
--- a/include/linux/phy_link_topology.h
+++ b/include/linux/phy_link_topology.h
@@ -36,6 +36,11 @@ struct phy_device_node {
 	struct phy_device *phy;
 };
 
+static inline bool phy_link_topo_empty(struct net_device *dev)
+{
+	return !dev->link_topo;
+}
+
 #if IS_ENABLED(CONFIG_PHYLIB)
 int phy_link_topo_add_phy(struct net_device *dev,
 			  struct phy_device *phy,
diff --git a/drivers/net/phy/phy_link_topology.c b/drivers/net/phy/phy_link_topology.c
index 1f1eb5d59b38..aed3b26c1674 100644
--- a/drivers/net/phy/phy_link_topology.c
+++ b/drivers/net/phy/phy_link_topology.c
@@ -10,6 +10,7 @@
 #include <linux/phy.h>
 #include <linux/rtnetlink.h>
 #include <linux/xarray.h>
+#include <net/netdev_lock.h>
 
 static int netdev_alloc_phy_link_topology(struct net_device *dev)
 {
@@ -35,6 +36,13 @@ int phy_link_topo_add_phy(struct net_device *dev,
 	struct phy_device_node *pdn;
 	int ret;
 
+	/* ethtool ops may run without rtnl_lock, and rtnl_lock is what
+	 * currently protects the PHY topology. No driver currently mixes
+	 * the two, flag if someone tries. See also ethnl_req_get_phydev().
+	 */
+	if (WARN_ON_ONCE(netdev_need_ops_lock(dev)))
+		return -EOPNOTSUPP;
+
 	if (!topo) {
 		ret = netdev_alloc_phy_link_topology(dev);
 		if (ret)
diff --git a/net/ethtool/netlink.c b/net/ethtool/netlink.c
index 902ff7d0a71d..a91cc4bc964f 100644
--- a/net/ethtool/netlink.c
+++ b/net/ethtool/netlink.c
@@ -226,11 +226,13 @@ struct phy_device *ethnl_req_get_phydev(const struct ethnl_req_info *req_info,
 {
 	struct phy_device *phydev;
 
-	ASSERT_RTNL();
-
 	if (!req_info->dev)
 		return NULL;
 
+	/* If there is no PHY in sight there's no need for assert locking */
+	if (!phy_link_topo_empty(req_info->dev))
+		ASSERT_RTNL();
+
 	if (!req_info->phy_index)
 		return req_info->dev->phydev;
 
diff --git a/net/ethtool/phy.c b/net/ethtool/phy.c
index ddc6eab701ed..018b0412be86 100644
--- a/net/ethtool/phy.c
+++ b/net/ethtool/phy.c
@@ -78,7 +78,6 @@ static int phy_prepare_data(const struct ethnl_req_info *req_info,
 	struct phy_device *phydev;
 	int ret;
 
-	/* RTNL is held by the caller */
 	phydev = ethnl_req_get_phydev(req_info, tb, ETHTOOL_A_PHY_HEADER,
 				      info->extack);
 	if (IS_ERR_OR_NULL(phydev))
-- 
2.54.0