Re: [RFC PATCH net-next] net: create a NETDEV_ETH_IOCTL notifier for DSA to reject PTP on DSA master

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 18 Mar 2022 00:50:35 +0200
Vladimir Oltean <vladimir.oltean@nxp.com> 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?