net: phy: smsc: Disable Energy Detect Power-Down in interrupt mode

Simon reports that if two LAN9514 USB adapters are directly connected
without an intermediate switch, the link fails to come up and link LEDs
remain dark.  The issue was introduced by commit 1ce8b372 ("usbnet:
smsc95xx: Forward PHY interrupts to PHY driver to avoid polling").

The PHY suffers from a known erratum wherein link detection becomes
unreliable if Energy Detect Power-Down is used.  In poll mode, the
driver works around the erratum by briefly disabling EDPD for 640 msec
to detect a neighbor, then re-enabling it to save power.

In interrupt mode, no interrupt is signaled if EDPD is used by both link
partners, so it must not be enabled at all.

We'll recoup the power savings by enabling SUSPEND1 mode on affected
LAN95xx chips in a forthcoming commit.

Fixes: 1ce8b372 ("usbnet: smsc95xx: Forward PHY interrupts to PHY driver to avoid polling")
Reported-by: Simon Han <z.han@kunbus.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/439a3f3168c2f9d44b5fd9bb8d2b551711316be6.1655714438.git.lukas@wunner.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/smsc.c

@@ -110,7 +110,7 @@ static int smsc_phy_config_init(struct phy_device *phydev)
 	struct smsc_phy_priv *priv = phydev->priv;
 	int rc;
 
-	if (!priv->energy_enable)
+	if (!priv->energy_enable || phydev->irq != PHY_POLL)
 		return 0;
 
 	rc = phy_read(phydev, MII_LAN83C185_CTRL_STATUS);
@@ -210,6 +210,8 @@ static int lan95xx_config_aneg_ext(struct phy_device *phydev)
  * response on link pulses to detect presence of plugged Ethernet cable.
  * The Energy Detect Power-Down mode is enabled again in the end of procedure to
  * save approximately 220 mW of power if cable is unplugged.
+ * The workaround is only applicable to poll mode. Energy Detect Power-Down may
+ * not be used in interrupt mode lest link change detection becomes unreliable.
  */
 static int lan87xx_read_status(struct phy_device *phydev)
 {
@@ -217,7 +219,7 @@ static int lan87xx_read_status(struct phy_device *phydev)
 
 	int err = genphy_read_status(phydev);
 
-	if (!phydev->link && priv->energy_enable) {
+	if (!phydev->link && priv->energy_enable && phydev->irq == PHY_POLL) {
 		/* Disable EDPD to wake up PHY */
 		int rc = phy_read(phydev, MII_LAN83C185_CTRL_STATUS);
 		if (rc < 0)