net: phylink: add phylink_limit_mac_speed()

Add a function which can be used to limit the phylink MAC capabilities
to an upper speed limit.
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qZAX3-005pTi-K1@rmk-PC.armlinux.org.ukSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/phylink.c

@@ -426,6 +426,24 @@ static struct {
 	{ MAC_10HD,     SPEED_10,     DUPLEX_HALF },
 };
 
+/**
+ * phylink_limit_mac_speed - limit the phylink_config to a maximum speed
+ * @config: pointer to a &struct phylink_config
+ * @max_speed: maximum speed
+ *
+ * Mask off MAC capabilities for speeds higher than the @max_speed parameter.
+ * Any further motifications of config.mac_capabilities will override this.
+ */
+void phylink_limit_mac_speed(struct phylink_config *config, u32 max_speed)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(phylink_caps_params) &&
+		    phylink_caps_params[i].speed > max_speed; i++)
+		config->mac_capabilities &= ~phylink_caps_params[i].mask;
+}
+EXPORT_SYMBOL_GPL(phylink_limit_mac_speed);
+
 /**
  * phylink_cap_from_speed_duplex - Get mac capability from speed/duplex
  * @speed: the speed to search for

include/linux/phylink.h

@@ -223,6 +223,8 @@ struct phylink_config {
 	unsigned long mac_capabilities;
 };
 
+void phylink_limit_mac_speed(struct phylink_config *config, u32 max_speed);
+
 /**
  * struct phylink_mac_ops - MAC operations structure.
  * @validate: Validate and update the link configuration.