net: dsa: mv88e6xxx: propperly shutdown PPU re-enable timer on destroy

The mv88e6xxx has an internal PPU that polls PHY state. If we want to
access the internal PHYs, we need to disable the PPU first. Because
that is a slow operation, a 10ms timer is used to re-enable it,
canceled with every access, so bulk operations effectively only
disable it once and re-enable it some 10ms after the last access.

If a PHY is accessed and then the mv88e6xxx module is removed before
the 10ms are up, the PPU re-enable ends up accessing a dangling pointer.

This especially affects probing during bootup. The MDIO bus and PHY
registration may succeed, but registration with the DSA framework
may fail later on (e.g. because the CPU port depends on another,
very slow device that isn't done probing yet, returning -EPROBE_DEFER).
In this case, probe() fails, but the MDIO subsystem may already have
accessed the MIDO bus or PHYs, arming the timer.

This is fixed as follows:
- If probe fails after mv88e6xxx_phy_init(), make sure we also call
mv88e6xxx_phy_destroy() before returning
- In mv88e6xxx_remove(), make sure we do the teardown in the correct
order, calling mv88e6xxx_phy_destroy() after unregistering the
switch device.
- In mv88e6xxx_phy_destroy(), destroy both the timer and the work item
that the timer might schedule, synchronously waiting in case one of
the callbacks already fired and destroying the timer first, before
waiting for the work item.
- Access to the PPU is guarded by a mutex, the worker acquires it
with a mutex_trylock(), not proceeding with the expensive shutdown
if that fails. We grab the mutex in mv88e6xxx_phy_destroy() to make
sure the slow PPU shutdown is already done or won't even enter, when
we wait for the work item.

Fixes: 2e5f032095ff ("dsa: add support for the Marvell 88E6131 switch chip")
Signed-off-by: David Oberhollenzer <[e-mail]>
Reviewed-by: Vladimir Oltean <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/dsa/mv88e6xxx/chip.c

···
 	err = mv88e6xxx_switch_reset(chip);
 	mv88e6xxx_reg_unlock(chip);
 	if (err)
-		goto out;
+		goto out_phy;
 
 	if (np) {
 		chip->irq = of_irq_get(np, 0);
 		if (chip->irq == -EPROBE_DEFER) {
 			err = chip->irq;
-			goto out;
+			goto out_phy;
 		}
 	}
 
···
 	mv88e6xxx_reg_unlock(chip);
 
 	if (err)
-		goto out;
+		goto out_phy;
 
 	if (chip->info->g2_irqs > 0) {
 		err = mv88e6xxx_g2_irq_setup(chip);
···
 		mv88e6xxx_g1_irq_free(chip);
 	else
 		mv88e6xxx_irq_poll_free(chip);
+out_phy:
+	mv88e6xxx_phy_destroy(chip);
 out:
 	if (pdata)
 		dev_put(pdata->netdev);
···
 		mv88e6xxx_ptp_free(chip);
 	}
 
-	mv88e6xxx_phy_destroy(chip);
 	mv88e6xxx_unregister_switch(chip);
 
 	mv88e6xxx_g1_vtu_prob_irq_free(chip);
···
 		mv88e6xxx_g1_irq_free(chip);
 	else
 		mv88e6xxx_irq_poll_free(chip);
+
+	mv88e6xxx_phy_destroy(chip);
 }
 
 static void mv88e6xxx_shutdown(struct mdio_device *mdiodev)

drivers/net/dsa/mv88e6xxx/phy.c

···
 
 static void mv88e6xxx_phy_ppu_state_destroy(struct mv88e6xxx_chip *chip)
 {
+	mutex_lock(&chip->ppu_mutex);
 	del_timer_sync(&chip->ppu_timer);
+	cancel_work_sync(&chip->ppu_work);
+	mutex_unlock(&chip->ppu_mutex);
 }
 
 int mv88e6185_phy_ppu_read(struct mv88e6xxx_chip *chip, struct mii_bus *bus,