net: dsa: make dsa_master_ioctl() see through port_hwtstamp_get() shims

There are multi-generational drivers like mv88e6xxx which have code like
this:

int mv88e6xxx_port_hwtstamp_get(struct dsa_switch *ds, int port,
				struct ifreq *ifr)
{
	if (!chip->info->ptp_support)
		return -EOPNOTSUPP;

	...
}

DSA wants to deny PTP timestamping on the master if the switch supports
timestamping too. However it currently relies on the presence of the
port_hwtstamp_get() callback to determine PTP capability, and this
clearly does not work in that case (method is present but returns
-EOPNOTSUPP).

We should not deny PTP on the DSA master for those switches which truly
do not support hardware timestamping.

Create a dsa_port_supports_hwtstamp() method which actually probes for
support by calling port_hwtstamp_get() and seeing whether that returned
-EOPNOTSUPP or not.

Fixes: f685e609 ("net: dsa: Deny PTP on master if switch supports it")
Link: https://patchwork.kernel.org/project/netdevbpf/patch/20221110124345.3901389-1-festevam@gmail.com/Reported-by: Fabio Estevam <festevam@gmail.com>
Reported-by: Steffen Bätz <steffen@innosonix.de>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa_priv.h

@@ -210,6 +210,7 @@ static inline struct net_device *dsa_master_find_slave(struct net_device *dev,
 extern struct rtnl_link_ops dsa_link_ops __read_mostly;
 
 /* port.c */
+bool dsa_port_supports_hwtstamp(struct dsa_port *dp, struct ifreq *ifr);
 void dsa_port_set_tag_protocol(struct dsa_port *cpu_dp,
 			       const struct dsa_device_ops *tag_ops);
 int dsa_port_set_state(struct dsa_port *dp, u8 state, bool do_fast_age);

net/dsa/master.c

@@ -204,8 +204,7 @@ static int dsa_master_ioctl(struct net_device *dev, struct ifreq *ifr, int cmd)
 		 * switch in the tree that is PTP capable.
 		 */
 		list_for_each_entry(dp, &dst->ports, list)
-			if (dp->ds->ops->port_hwtstamp_get ||
-			    dp->ds->ops->port_hwtstamp_set)
+			if (dsa_port_supports_hwtstamp(dp, ifr))
 				return -EBUSY;
 		break;
 	}

net/dsa/port.c

@@ -110,6 +110,22 @@ static bool dsa_port_can_configure_learning(struct dsa_port *dp)
 	return !err;
 }
 
+bool dsa_port_supports_hwtstamp(struct dsa_port *dp, struct ifreq *ifr)
+{
+	struct dsa_switch *ds = dp->ds;
+	int err;
+
+	if (!ds->ops->port_hwtstamp_get || !ds->ops->port_hwtstamp_set)
+		return false;
+
+	/* "See through" shim implementations of the "get" method.
+	 * This will clobber the ifreq structure, but we will either return an
+	 * error, or the master will overwrite it with proper values.
+	 */
+	err = ds->ops->port_hwtstamp_get(ds, dp->index, ifr);
+	return err != -EOPNOTSUPP;
+}
+
 int dsa_port_set_state(struct dsa_port *dp, u8 state, bool do_fast_age)
 {
 	struct dsa_switch *ds = dp->ds;