[RFC PATCH net 4/5] net: dsa: microchip: ksz8863: be compatible with masters which unregister on shutdown

[Vladimir Oltean]

Since commit 2f1e8ea726e9 ("net: dsa: link interfaces with the DSA
master to get rid of lockdep warnings"), DSA gained a requirement which
it did not fulfill, which is to unlink itself from the DSA master at
shutdown time.

Since the Microchip sub-driver for KSZ8863 was introduced after the bad
commit, it has never worked with DSA masters which decide to unregister
their net_device on shutdown, effectively hanging the reboot process.
To fix that, we need to call dsa_switch_shutdown.

Since this driver expects the MDIO bus to be backed by mdio_bitbang, I
don't think there is currently any MDIO bus driver which implements its
->shutdown by redirecting it to ->remove, but in any case, to be
compatible with that pattern, it is necessary to implement an "if this
then not that" scheme, to avoid ->remove and ->shutdown from being
called both for the same struct device.

Fixes: 60a364760002 ("net: dsa: microchip: Add Microchip KSZ8863 SMI based driver support")
Link: https://lore.kernel.org/netdev/20210909095324.12978-1-LinoSanfilippo@xxxxxx/
Reported-by: Lino Sanfilippo <LinoSanfilippo@xxxxxx>
Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
---
 drivers/net/dsa/microchip/ksz8863_smi.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/net/dsa/microchip/ksz8863_smi.c b/drivers/net/dsa/microchip/ksz8863_smi.c
index 11293485138c..5883fa7edda2 100644
--- a/drivers/net/dsa/microchip/ksz8863_smi.c
+++ b/drivers/net/dsa/microchip/ksz8863_smi.c
@@ -191,6 +191,18 @@ static void ksz8863_smi_remove(struct mdio_device *mdiodev)
 
 	if (dev)
 		ksz_switch_remove(dev);
+
+	dev_set_drvdata(&mdiodev->dev, NULL);
+}
+
+static void ksz8863_smi_shutdown(struct mdio_device *mdiodev)
+{
+	struct ksz_device *dev = dev_get_drvdata(&mdiodev->dev);
+
+	if (dev)
+		dsa_switch_shutdown(dev->ds);
+
+	dev_set_drvdata(&mdiodev->dev, NULL);
 }
 
 static const struct of_device_id ksz8863_dt_ids[] = {
@@ -203,6 +215,7 @@ MODULE_DEVICE_TABLE(of, ksz8863_dt_ids);
 static struct mdio_driver ksz8863_driver = {
 	.probe	= ksz8863_smi_probe,
 	.remove	= ksz8863_smi_remove,
+	.shutdown = ksz8863_smi_shutdown,
 	.mdiodrv.driver = {
 		.name	= "ksz8863-switch",
 		.of_match_table = ksz8863_dt_ids,
-- 
2.25.1