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>