mlxsw: spectrum_ptp: Support SIOCGHWTSTAMP, SIOCSHWTSTAMP ioctls

The SIOCSHWTSTAMP ioctl configures HW timestamping on a given port. In
Spectrum-2 and above, each packet gets time stamp by default, but in
order to provide an accurate time stamp, software should configure to
update the correction field. In addition, the PTP traps are not enabled
by default, software should enable it per port or for all ports.

The switch behaves like a transparent clock between CPU port and each
front panel port. If ingress correction is set on a port for a given packet
type, then when such a packet is received via the port, the current time
stamp is subtracted from the correction field. If egress correction is set
on a port for a given packet type, then when such a packet is transmitted
via the port, the current time stamp is added to the correction field.

The result is that as the packet ingresses through a port with ingress
correction enabled, and egresses through a port with egress correction
enabled, the PTP correction field is updated to reflect the time that the
packet spent in the ASIC.

This can be used to update the correction field of trapped packets by
enabling ingress correction on a port where time stamping was enabled,
and egress correction on the CPU port. Similarly, for packets transmitted
from the host, ingress correction should be enabled on the CPU port, and
egress correction on a front-panel port.

However, since the correction fields will be updated for all PTP packets
crossing the CPU port, in order not to mangle the correction field, the
front panel port involved in the packet transfer must have the
corresponding correction enabled as well.

Therefore, when HW timestamping is enabled on at least one port, we have
to configure hardware to update the correction field and trap PTP event
packets on all ports.

Add reference count as part of 'struct mlxsw_sp_ptp_state', to maintain
how many ports use HW timestamping. Handle the correction field
configuration only when the first port enables time stamping and when the
last port disables time stamping. Store the configuration as part of
'struct mlxsw_sp_ptp_state', as it is global for all ports.

The SIOCGHWTSTAMP ioctl is a getter for the current configuration,
implement it and use the global configuration.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>