net: phy: phylink: fix DSA mac_select_pcs() introduction

Vladimir Oltean reports that probing on DSA drivers that aren't yet
populating supported_interfaces now fails. Fix this by allowing
phylink to detect whether DSA actually provides an underlying
mac_select_pcs() implementation.

Reported-by: Vladimir Oltean <[e-mail]>
Fixes: bde018222c6b ("net: dsa: add support for phylink mac_select_pcs()")
Signed-off-by: Russell King (Oracle) <[e-mail]>
Tested-by: Vladimir Oltean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/phy/phylink.c

···
 	struct work_struct resolve;
 
 	bool mac_link_dropped;
+	bool using_mac_select_pcs;
 
 	struct sfp_bus *sfp_bus;
 	bool sfp_may_have_phy;
···
 	int ret;
 
 	/* Get the PCS for this interface mode */
-	if (pl->mac_ops->mac_select_pcs) {
+	if (pl->using_mac_select_pcs) {
 		pcs = pl->mac_ops->mac_select_pcs(pl->config, state->interface);
 		if (IS_ERR(pcs))
 			return PTR_ERR(pcs);
···
 
 	phylink_dbg(pl, "major config %s\n", phy_modes(state->interface));
 
-	if (pl->mac_ops->mac_select_pcs) {
+	if (pl->using_mac_select_pcs) {
 		pcs = pl->mac_ops->mac_select_pcs(pl->config, state->interface);
 		if (IS_ERR(pcs)) {
 			phylink_err(pl,
···
 			       phy_interface_t iface,
 			       const struct phylink_mac_ops *mac_ops)
 {
+	bool using_mac_select_pcs = false;
 	struct phylink *pl;
 	int ret;
 
-	/* Validate the supplied configuration */
 	if (mac_ops->mac_select_pcs &&
+	    mac_ops->mac_select_pcs(config, PHY_INTERFACE_MODE_NA) !=
+	      ERR_PTR(-EOPNOTSUPP))
+		using_mac_select_pcs = true;
+
+	/* Validate the supplied configuration */
+	if (using_mac_select_pcs &&
 	    phy_interface_empty(config->supported_interfaces)) {
 		dev_err(config->dev,
 			"phylink: error: empty supported_interfaces but mac_select_pcs() method present\n");
···
 		return ERR_PTR(-EINVAL);
 	}
 
+	pl->using_mac_select_pcs = using_mac_select_pcs;
 	pl->phy_state.interface = iface;
 	pl->link_interface = iface;
 	if (iface == PHY_INTERFACE_MODE_MOCA)

net/dsa/port.c

···
 				phy_interface_t interface)
 {
 	struct dsa_port *dp = container_of(config, struct dsa_port, pl_config);
+	struct phylink_pcs *pcs = ERR_PTR(-EOPNOTSUPP);
 	struct dsa_switch *ds = dp->ds;
-	struct phylink_pcs *pcs = NULL;
 
 	if (ds->ops->phylink_mac_select_pcs)
 		pcs = ds->ops->phylink_mac_select_pcs(ds, dp->index, interface);