net: phy: Warn about incorrect mdio_bus_phy_resume() state

Calling mdio_bus_phy_resume() with neither the PHY state machine set to
PHY_HALTED nor phydev->mac_managed_pm set to true is a good indication
that we can produce a race condition looking like this:

CPU0						CPU1
bcmgenet_resume
 -> phy_resume
   -> phy_init_hw
 -> phy_start
   -> phy_resume
                                                phy_start_aneg()
mdio_bus_phy_resume
 -> phy_resume
    -> phy_write(..., BMCR_RESET)
     -> usleep()                                  -> phy_read()

with the phy_resume() function triggering a PHY behavior that might have
to be worked around with (see bf8bfc4 ("net: phy: broadcom: Fix
brcm_fet_config_init()") for instance) that ultimately leads to an error
reading from the PHY.

Fixes: fba863b ("net: phy: make PHY PM ops a no-op if MAC driver manages PHY PM")
Signed-off-by: Florian Fainelli <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/net/phy/phy_device.c

@@ -316,6 +316,12 @@ static __maybe_unused int mdio_bus_phy_resume(struct device *dev)
 
 	phydev->suspended_by_mdio_bus = 0;
 
+	/* If we managed to get here with the PHY state machine in a state other
+	 * than PHY_HALTED this is an indication that something went wrong and
+	 * we should most likely be using MAC managed PM and we are not.
+	 */
+	WARN_ON(phydev->state != PHY_HALTED && !phydev->mac_managed_pm);
+
 	ret = phy_init_hw(phydev);
 	if (ret < 0)
 		return ret;