ethtool: netlink: do not return SQI value if link is down

Do not attach SQI value if link is down. "SQI values are only valid if
link-up condition is present" per OpenAlliance specification of
100Base-T1 Interoperability Test suite [1]. The same rule would apply
for other link types.

[1] https://opensig.org/automotive-ethernet-specifications/#

Fixes: 8066021 ("ethtool: provide UAPI for PHY Signal Quality Index (SQI)")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Woojung Huh <woojung.huh@microchip.com>
Link: https://patch.msgid.link/20240709061943.729381-1-o.rempel@pengutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: olerem

net/ethtool/linkstate.c

@@ -37,6 +37,8 @@ static int linkstate_get_sqi(struct net_device *dev)
 	mutex_lock(&phydev->lock);
 	if (!phydev->drv || !phydev->drv->get_sqi)
 		ret = -EOPNOTSUPP;
+	else if (!phydev->link)
+		ret = -ENETDOWN;
 	else
 		ret = phydev->drv->get_sqi(phydev);
 	mutex_unlock(&phydev->lock);
@@ -55,13 +57,26 @@ static int linkstate_get_sqi_max(struct net_device *dev)
 	mutex_lock(&phydev->lock);
 	if (!phydev->drv || !phydev->drv->get_sqi_max)
 		ret = -EOPNOTSUPP;
+	else if (!phydev->link)
+		ret = -ENETDOWN;
 	else
 		ret = phydev->drv->get_sqi_max(phydev);
 	mutex_unlock(&phydev->lock);
 
 	return ret;
 };
 
+static bool linkstate_sqi_critical_error(int sqi)
+{
+	return sqi < 0 && sqi != -EOPNOTSUPP && sqi != -ENETDOWN;
+}
+
+static bool linkstate_sqi_valid(struct linkstate_reply_data *data)
+{
+	return data->sqi >= 0 && data->sqi_max >= 0 &&
+	       data->sqi <= data->sqi_max;
+}
+
 static int linkstate_get_link_ext_state(struct net_device *dev,
 					struct linkstate_reply_data *data)
 {
@@ -93,12 +108,12 @@ static int linkstate_prepare_data(const struct ethnl_req_info *req_base,
 	data->link = __ethtool_get_link(dev);
 
 	ret = linkstate_get_sqi(dev);
-	if (ret < 0 && ret != -EOPNOTSUPP)
+	if (linkstate_sqi_critical_error(ret))
 		goto out;
 	data->sqi = ret;
 
 	ret = linkstate_get_sqi_max(dev);
-	if (ret < 0 && ret != -EOPNOTSUPP)
+	if (linkstate_sqi_critical_error(ret))
 		goto out;
 	data->sqi_max = ret;
 
@@ -136,11 +151,10 @@ static int linkstate_reply_size(const struct ethnl_req_info *req_base,
 	len = nla_total_size(sizeof(u8)) /* LINKSTATE_LINK */
 		+ 0;
 
-	if (data->sqi != -EOPNOTSUPP)
-		len += nla_total_size(sizeof(u32));
-
-	if (data->sqi_max != -EOPNOTSUPP)
-		len += nla_total_size(sizeof(u32));
+	if (linkstate_sqi_valid(data)) {
+		len += nla_total_size(sizeof(u32)); /* LINKSTATE_SQI */
+		len += nla_total_size(sizeof(u32)); /* LINKSTATE_SQI_MAX */
+	}
 
 	if (data->link_ext_state_provided)
 		len += nla_total_size(sizeof(u8)); /* LINKSTATE_EXT_STATE */
@@ -164,13 +178,14 @@ static int linkstate_fill_reply(struct sk_buff *skb,
 	    nla_put_u8(skb, ETHTOOL_A_LINKSTATE_LINK, !!data->link))
 		return -EMSGSIZE;
 
-	if (data->sqi != -EOPNOTSUPP &&
-	    nla_put_u32(skb, ETHTOOL_A_LINKSTATE_SQI, data->sqi))
-		return -EMSGSIZE;
+	if (linkstate_sqi_valid(data)) {
+		if (nla_put_u32(skb, ETHTOOL_A_LINKSTATE_SQI, data->sqi))
+			return -EMSGSIZE;
 
-	if (data->sqi_max != -EOPNOTSUPP &&
-	    nla_put_u32(skb, ETHTOOL_A_LINKSTATE_SQI_MAX, data->sqi_max))
-		return -EMSGSIZE;
+		if (nla_put_u32(skb, ETHTOOL_A_LINKSTATE_SQI_MAX,
+				data->sqi_max))
+			return -EMSGSIZE;
+	}
 
 	if (data->link_ext_state_provided) {
 		if (nla_put_u8(skb, ETHTOOL_A_LINKSTATE_EXT_STATE,