net: cpsw: convert to ndo_hwtstamp_get() and ndo_hwtstamp_set()

New timestamping API was introduced in commit 66f7223039c0 ("net: add
NDOs for configuring hardware timestamping") from kernel v6.6. It is
time to convert the two cpsw drivers to the new API, so that the
ndo_eth_ioctl() path can be removed completely.

The cpsw_hwtstamp_get() and cpsw_hwtstamp_set() methods (and their shim
definitions, for the case where CONFIG_TI_CPTS is not enabled) must have
their prototypes adjusted.

These methods are used by two drivers (cpsw and cpsw_new), with vastly
different configurations:
- cpsw has two operating modes:
  - "dual EMAC" - enabled through the "dual_emac" device tree property -
    creates one net_device per EMAC / slave interface (but there is no
    bridging offload)
  - "switch mode" - default - there is a single net_device, with two
    EMACs/slaves behind it (and switching between them happens
    unbeknownst to the network stack).
- cpsw_new always registers one net_device for each EMAC which doesn't
  have status = "disabled". In terms of switching, it has two modes:
  - "dual EMAC": default, no switching between ports, no switchdev
    offload.
  - "switch mode": enabled through the "switch_mode" devlink parameter,
    offloads the Linux bridge through switchdev

Essentially, in 3 out of 4 operating modes, there is a bijective
relation between the net_device and the slave. Timestamping can thus be
configured on individual slaves. But in the "switch mode" of the cpsw
driver, ndo_eth_ioctl() targets a single slave, designated using the
"active_slave" device tree property.

To deal with these different cases, the common portion of the drivers,
cpsw_priv.c, has the cpsw_slave_index() function pointer, set to
separate, identically named cpsw_slave_index_priv() by the 2 drivers.

This is all relevant because cpsw_ndo_ioctl() has the old-style
phy_has_hwtstamp() logic which lets the PHY handle the timestamping
ioctls. Normally, that logic should be obsoleted by the more complex
logic in the core, which permits dynamically selecting the timestamp
provider - see dev_set_hwtstamp_phylib().

But I have doubts as to how this works for the "switch mode" of the dual
EMAC driver, because the core logic only engages if the PHY is visible
through ndev->phydev (this is set by phy_attach_direct()).

In cpsw.c, we have:
cpsw_ndo_open()
-> for_each_slave(priv, cpsw_slave_open, priv); // continues on errors
   -> of_phy_connect()
      -> phy_connect_direct()
         -> phy_attach_direct()
   OR
   -> phy_connect()
      -> phy_connect_direct()
         -> phy_attach_direct()

The problem for "switch mode" is that the behavior of phy_attach_direct()
called twice in a row for the same net_device (once for each slave) is
probably undefined.

For sure it will overwrite dev->phydev. I don't see any explicit error
checks for this case, and even if there were, the for_each_slave() call
makes them non-fatal to cpsw_ndo_open() anyway.

I have no idea what is the extent to which this provides a usable
result, but the point is: only the last attached PHY will be visible
in dev->phydev, and this may well be a different PHY than
cpsw->slaves[slave_no].phy for the "active_slave".

In dual EMAC mode, as well as in cpsw_new, this should not be a problem.
I don't know whether PHY timestamping is a use case for the cpsw "switch
mode" as well, and I hope that there isn't, because for the sake of
simplicity, I've decided to deliberately break that functionality, by
refusing all PHY timestamping. Keeping it would mean blocking the old
API from ever being removed. In the new dev_set_hwtstamp_phylib() API,
it is not possible to operate on a phylib PHY other than dev->phydev,
and I would very much prefer not adding that much complexity for bizarre
driver decisions.

Final point about the cpsw_hwtstamp_get() conversion: we don't need to
propagate the unnecessary "config.flags = 0;", because dev_get_hwtstamp()
provides a zero-initialized struct kernel_hwtstamp_config.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250512114422.4176010-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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