Currently mlxsw driver configures one global PTP configuration for all
ports. The reason is that the switch behaves like a transparent clock
between CPU port and front-panel ports. When time stamp is enabled in
any port, the hardware is configured to update the correction field. The
fact that the configuration of CPU port affects all the ports, makes the
correction field update to be global for all ports. Otherwise, user will
see odd values in the correction field, as the switch will update the
correction field in the CPU port, but not in all the front-panel ports.

The CPU port is relevant in both RX and TX, so to avoid problematic
configuration, forbid PTP enablement only in one direction, i.e., only in
RX or TX.

Without the change:
$ hwstamp_ctl -i swp1 -r 12 -t 0
current settings:
tx_type 0
rx_filter 0
new settings:
tx_type 0
rx_filter 2
$ echo $?
0

With the change:
$ hwstamp_ctl -i swp1 -r 12 -t 0
current settings:
tx_type 1
rx_filter 2
SIOCSHWTSTAMP failed: Invalid argument

Fixes: 08ef8bc8 ("mlxsw: spectrum_ptp: Support SIOCGHWTSTAMP, SIOCSHWTSTAMP ioctls")
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>