[PATCH net-next 4/7] usbnet: smsc95xx: Avoid link settings race on interrupt reception

[Lukas Wunner <lukas@xxxxxxxxx>]

When a PHY interrupt is signaled, the SMSC LAN95xx driver updates the
MAC full duplex mode and PHY flow control registers based on cached data
in struct phy_device:

  smsc95xx_status()                 # raises EVENT_LINK_RESET
    usbnet_deferred_kevent()
      smsc95xx_link_reset()         # uses cached data in phydev

Simultaneously, phylib polls link status once per second and updates
that cached data:

  phy_state_machine()
    phy_check_link_status()
      phy_read_status()
        lan87xx_read_status()
          genphy_read_status()      # updates cached data in phydev

If smsc95xx_link_reset() wins the race against genphy_read_status(),
the registers may be updated based on stale data.

E.g. if the link was previously down, phydev->duplex is set to
DUPLEX_UNKNOWN and that's what smsc95xx_link_reset() will use, even
though genphy_read_status() may update it to DUPLEX_FULL afterwards.

PHY interrupts are currently only enabled on suspend to trigger wakeup,
so the impact of the race is limited, but we're about to enable them
perpetually.

Avoid the race by delaying execution of smsc95xx_link_reset() until
phy_state_machine() has done its job and calls back via
smsc95xx_handle_link_change().

Signaling EVENT_LINK_RESET on wakeup is not necessary because phylib
picks up link status changes through polling.  So drop the declaration
of a ->link_reset() callback.

Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
---
 drivers/net/usb/smsc95xx.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index 6c37c7adde1b..36abfaeae3c6 100644
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -566,7 +566,7 @@ static int smsc95xx_phy_update_flowcontrol(struct usbnet *dev)
 	return smsc95xx_write_reg(dev, AFC_CFG, afc_cfg);
 }
 
-static int smsc95xx_link_reset(struct usbnet *dev)
+static void smsc95xx_mac_update_fullduplex(struct usbnet *dev)
 {
 	struct smsc95xx_priv *pdata = dev->driver_priv;
 	unsigned long flags;
@@ -583,14 +583,14 @@ static int smsc95xx_link_reset(struct usbnet *dev)
 	spin_unlock_irqrestore(&pdata->mac_cr_lock, flags);
 
 	ret = smsc95xx_write_reg(dev, MAC_CR, pdata->mac_cr);
-	if (ret < 0)
-		return ret;
+	if (ret < 0) {
+		netdev_warn(dev->net, "Error updating MAC full duplex mode\n");
+		return;
+	}
 
 	ret = smsc95xx_phy_update_flowcontrol(dev);
 	if (ret < 0)
 		netdev_warn(dev->net, "Error updating PHY flow control\n");
-
-	return ret;
 }
 
 static void smsc95xx_status(struct usbnet *dev, struct urb *urb)
@@ -607,7 +607,7 @@ static void smsc95xx_status(struct usbnet *dev, struct urb *urb)
 	netif_dbg(dev, link, dev->net, "intdata: 0x%08X\n", intdata);
 
 	if (intdata & INT_ENP_PHY_INT_)
-		usbnet_defer_kevent(dev, EVENT_LINK_RESET);
+		;
 	else
 		netdev_warn(dev->net, "unexpected interrupt, intdata=0x%08X\n",
 			    intdata);
@@ -1070,6 +1070,7 @@ static void smsc95xx_handle_link_change(struct net_device *net)
 	struct usbnet *dev = netdev_priv(net);
 
 	phy_print_status(net->phydev);
+	smsc95xx_mac_update_fullduplex(dev);
 	usbnet_defer_kevent(dev, EVENT_LINK_CHANGE);
 }
 
@@ -1975,7 +1976,6 @@ static const struct driver_info smsc95xx_info = {
 	.description	= "smsc95xx USB 2.0 Ethernet",
 	.bind		= smsc95xx_bind,
 	.unbind		= smsc95xx_unbind,
-	.link_reset	= smsc95xx_link_reset,
 	.reset		= smsc95xx_reset,
 	.check_connect	= smsc95xx_start_phy,
 	.stop		= smsc95xx_stop,
-- 
2.35.2