ethtool: add PHY Fast Link Down support

This adds support for Fast Link Down as new PHY tunable.
Fast Link Down reduces the time until a link down event is reported
for 1000BaseT. According to the standard it's 750ms what is too long
for several use cases.

v2:
- add comment describing the constants
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/ethtool.h

@@ -252,9 +252,17 @@ struct ethtool_tunable {
 #define DOWNSHIFT_DEV_DEFAULT_COUNT	0xff
 #define DOWNSHIFT_DEV_DISABLE		0
 
+/* Time in msecs after which link is reported as down
+ * 0 = lowest time supported by the PHY
+ * 0xff = off, link down detection according to standard
+ */
+#define ETHTOOL_PHY_FAST_LINK_DOWN_ON	0
+#define ETHTOOL_PHY_FAST_LINK_DOWN_OFF	0xff
+
 enum phy_tunable_id {
 	ETHTOOL_PHY_ID_UNSPEC,
 	ETHTOOL_PHY_DOWNSHIFT,
+	ETHTOOL_PHY_FAST_LINK_DOWN,
 	/*
 	 * Add your fresh new phy tunable attribute above and remember to update
 	 * phy_tunable_strings[] in net/core/ethtool.c

net/core/ethtool.c

@@ -136,6 +136,7 @@ static const char
 phy_tunable_strings[__ETHTOOL_PHY_TUNABLE_COUNT][ETH_GSTRING_LEN] = {
 	[ETHTOOL_ID_UNSPEC]     = "Unspec",
 	[ETHTOOL_PHY_DOWNSHIFT]	= "phy-downshift",
+	[ETHTOOL_PHY_FAST_LINK_DOWN] = "phy-fast-link-down",
 };
 
 static int ethtool_get_features(struct net_device *dev, void __user *useraddr)
@@ -2432,6 +2433,7 @@ static int ethtool_phy_tunable_valid(const struct ethtool_tunable *tuna)
 {
 	switch (tuna->id) {
 	case ETHTOOL_PHY_DOWNSHIFT:
+	case ETHTOOL_PHY_FAST_LINK_DOWN:
 		if (tuna->len != sizeof(u8) ||
 		    tuna->type_id != ETHTOOL_TUNABLE_U8)
 			return -EINVAL;