[PATCH net] net: phy: Don't assume loopback is supported

[Jose Abreu <jose.abreu@synopsys.com>]

Some PHYs may not support loopback mode so we need to check if register
is read-only.

Fixes: f0f9b4ed2338 ("net: phy: Add phy loopback support in net phy framework")
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Joao Pinto <joao.pinto@synopsys.com>
---
 drivers/net/phy/phy_device.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index 49fdd1ee798e..a749639d98c3 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c
@@ -1918,8 +1918,24 @@ EXPORT_SYMBOL(genphy_resume);
 
 int genphy_loopback(struct phy_device *phydev, bool enable)
 {
-	return phy_modify(phydev, MII_BMCR, BMCR_LOOPBACK,
-			  enable ? BMCR_LOOPBACK : 0);
+	int ret;
+
+	ret = phy_modify(phydev, MII_BMCR, BMCR_LOOPBACK,
+			 enable ? BMCR_LOOPBACK : 0);
+	if (ret < 0)
+		return ret;
+
+	ret = phy_read(phydev, MII_BMCR);
+	if (ret < 0)
+		return ret;
+
+	if (enable) {
+		if (ret & BMCR_LOOPBACK)
+			return 0;
+		return -EOPNOTSUPP;
+	}
+
+	return 0;
 }
 EXPORT_SYMBOL(genphy_loopback);
 
-- 
2.7.4