[PATCH net v2] net: phylink: unwind the PHY binding when bringup fails late

phylink_bringup_phy() records the PHY in pl->phydev before its last
fallible step: on a MAC whose phylink ops implement LPI,
phy_eee_rx_clock_stop() can fail with a real MDIO error. The callers
unwind with phy_detach(), which knows nothing about pl->phydev, so a
pointer to a PHY that is no longer attached outlives the failed
connect.

What that costs depends on how the caller got here.
phylink_connect_phy() goes through phylink_attach_phy(), which refuses
to attach while pl->phydev is set, turning a transient MDIO error into
a permanent -EBUSY. The SFP path is worse than that: sfp_sm_probe_phy()
answers the failure with phy_device_remove() and phy_device_free(), and
it assigns sfp->mod_phy only past that error return, so nothing clears
pl->phydev and it is left pointing at a freed phy_device that
phylink_resolve() and the ethtool helpers go on reading.
phylink_fwnode_phy_connect() has no such check, so a later connect
overwrites the stale pointer and hides the problem. A disconnect does
not: phylink_disconnect_phy() hands that pointer to phy_disconnect(),
and the second phy_detach() on the same PHY drops references the first
one already released.

Found while making a DSA port survive a PHY whose driver arrives after
the switch probes: keeping the port across a failed connect and
retrying is what makes this window reachable.

Clear the binding on the failure path. This is the same operation
phylink_disconnect_phy() performs, so both now share a helper. The
PHY-side fields are left to phy_detach(), which every caller already
runs on this path.

Fixes: 03abf2a7c654 ("net: phylink: add EEE management")
Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <redacted>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
---

Notes:
    v2: dropped the comment above phylink_clear_phydev(); phylink states no
    lock requirements in comments and leaves its static helpers uncommented,
    as Andrew pointed out. Added a sentence on how the defect was found. The
    code is identical to v1, so the Reviewed-by is kept.
    v1: https://lore.kernel.org/netdev/20260918015109.2518797-1-f@lex.la/ (local)
    
    Split out of the [PATCH net v7 0/2] pair (20260909204306.2374562-1-f@lex.la):
    2/2 is being reworked per review, and this patch has nothing left to wait
    for.
    
    Reviewed-by was given on v2 of the pair (net-next v2 1/2), lore message-id
    006ad9d6-b51b-46ed-8c5d-3649b70a4f62@lunn.ch.

 drivers/net/phy/phylink.c | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
index a1458da8111b..0e59b7ce8b7a 100644
--- a/drivers/net/phy/phylink.c
+++ b/drivers/net/phy/phylink.c

@@ -2085,6 +2085,17 @@ static int phylink_validate_phy(struct phylink *pl, struct phy_device *phy,
 	return phylink_validate(pl, supported, state);
 }
 
+static void phylink_clear_phydev(struct phylink *pl, struct phy_device *phy)
+{
+	mutex_lock(&phy->lock);
+	mutex_lock(&pl->state_mutex);
+	pl->phydev = NULL;
+	pl->phy_enable_tx_lpi = false;
+	pl->mac_tx_clk_stop = false;
+	mutex_unlock(&pl->state_mutex);
+	mutex_unlock(&phy->lock);
+}
+
 static int phylink_bringup_phy(struct phylink *pl, struct phy_device *phy,
 			       phy_interface_t interface)
 {

@@ -2199,6 +2210,12 @@ static int phylink_bringup_phy(struct phylink *pl, struct phy_device *phy,
 	if (ret == 0 && phy_interrupt_is_valid(phy))
 		phy_request_interrupt(phy);
 
+	if (ret) {
+		mutex_lock(&pl->phydev_mutex);
+		phylink_clear_phydev(pl, phy);
+		mutex_unlock(&pl->phydev_mutex);
+	}
+
 	return ret;
 }
 

@@ -2349,15 +2366,8 @@ void phylink_disconnect_phy(struct phylink *pl)
 
 	mutex_lock(&pl->phydev_mutex);
 	phy = pl->phydev;
-	if (phy) {
-		mutex_lock(&phy->lock);
-		mutex_lock(&pl->state_mutex);
-		pl->phydev = NULL;
-		pl->phy_enable_tx_lpi = false;
-		pl->mac_tx_clk_stop = false;
-		mutex_unlock(&pl->state_mutex);
-		mutex_unlock(&phy->lock);
-	}
+	if (phy)
+		phylink_clear_phydev(pl, phy);
 	mutex_unlock(&pl->phydev_mutex);
 
 	if (phy) {

-- 
2.53.0