net: phy: avoid NPE if read_page/write_page callbacks are not available

Currently there's a bug in the module subsystem [0] preventing load of
the PHY driver module on certain systems (as one symptom).
This results in a NPE on such systems for the following reason:
Instead of the correct PHY driver the genphy driver is loaded that
doesn't implement the read_page/write_page callbacks. Every call to
phy_read_paged() et al will result in a NPE therefore.

In parallel to fixing the root cause we should make sure that this one
and maybe similar issues in other subsystems don't result in a NPE
in phylib. So let's check for the callbacks before using them and warn
once if they are not available.

[0] https://marc.info/?t=157072642100001&r=1&w=2

Signed-off-by: Heiner Kallweit <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

drivers/net/phy/phy-core.c

@@ -697,11 +697,17 @@ EXPORT_SYMBOL_GPL(phy_modify_mmd);
 
 static int __phy_read_page(struct phy_device *phydev)
 {
+	if (WARN_ONCE(!phydev->drv->read_page, "read_page callback not available, PHY driver not loaded?\n"))
+		return -EOPNOTSUPP;
+
 	return phydev->drv->read_page(phydev);
 }
 
 static int __phy_write_page(struct phy_device *phydev, int page)
 {
+	if (WARN_ONCE(!phydev->drv->write_page, "write_page callback not available, PHY driver not loaded?\n"))
+		return -EOPNOTSUPP;
+
 	return phydev->drv->write_page(phydev, page);
 }