On Fri, 18 Mar 2022 00:50:35 +0200
Vladimir Oltean [off-list ref] wrote:

The fact that PTP 2-step TX timestamping is deeply broken on DSA
switches if the master also timestamps the same packets is well
documented by commit f685e609a301 ("net: dsa: Deny PTP on master if
switch supports it"). We attempt to help the users avoid shooting
themselves in the foot by making DSA reject the timestamping ioctls on
an interface that is a DSA master, and the switch tree beneath it
contains switches which are aware of PTP.

The only problem is that there isn't an established way of intercepting
ndo_eth_ioctl calls, so DSA creates avoidable burden upon the network
stack by creating a struct dsa_netdevice_ops with overlaid function
pointers that are manually checked from the relevant call sites. There
used to be 2 such dsa_netdevice_ops, but now, ndo_eth_ioctl is the only
one left.

In fact, the underlying reason which is prompting me to make this change
is that I'd like to hide as many DSA data structures from public API as
I can. But struct net_device :: dsa_ptr is a struct dsa_port (which is a
huge structure), and I'd like to create a smaller structure. I'd like
struct dsa_netdevice_ops to not be a part of this, so this is how the
need to delete it arose.

The established way for unrelated modules to react on a net device event
is via netdevice notifiers. These have the advantage of loose coupling,
i.e. they work even when DSA is built as module, without resorting to
static inline functions (which cannot offer the desired data structure
encapsulation).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>

Why is this not using netlink?