MT-150266: net: fec: add direct MDIO bus access via SIOCGMIIREG/SIOCSMIIREG

[Overdr0ne]

MT-150266

This is the last remaining EVK patch from Audinate that applies to our kernel. Just like the other patches, the patch had no commit message, so I have had to infer what it was for exactly. As far as I can tell, the commit provides low level access to the fec phy for diagnostic purposes. However, I would not put it past Audinate to use an interface designed only for diagnostic purposes for production use. For this reason I would suggest trying it, perhaps for some of the link-local issues we were seeing where PTP/aipec were using the wrong interface ip range for multicast @AmeNote-Michael The commit message below was generated by Claude. I asked it to be particularly verbose for this one.

Add a fec_enet_ioctl() wrapper that services the SIOCGMIIREG and SIOCSMIIREG ioctls directly against fep->mii_bus, delegating all other commands to the standard phy_do_ioctl_running() handler.

The generic PHY ioctl path has two limitations for board bring-up and diagnostics:

• It requires an attached phydev. phy_do_ioctl() returns -ENODEV when ndev->phydev is NULL, which is the case when the FEC is wired to a DSA switch (SJA1105 on this platform) over a fixed-link rather than to a conventional PHY. In that configuration the MDIO bus cannot be reached from userspace at all.

• For an attached PHY it routes register writes through the PHY state machine, applying side effects (autoneg disable/restart, phy_init_hw) for BMCR/ADVERTISE/CTRL1000 writes. That is undesirable for raw register inspection and poking.

Servicing the two MII register commands directly against the bus gives side-effect-free access to any device address on the FEC MDIO bus, which userspace tooling relies on.

This reworks a patch distributed with the Audinate i.MX8MM EVK to fit the 6.6 driver, which no longer has a custom fec_enet_ioctl() (it wired phy_do_ioctl_running() directly). The original logged failures with KERN_ALERT and ignored read errors; this version returns the bus error to the caller and propagates read failures instead of reporting a bogus zero.

This is a diagnostic capability rather than a correctness fix. If it proves unused (verifiable at runtime by stracing for SIOC[GS]MIIREG) or problematic, it can be reverted without affecting normal operation.

dev QA: @AmeNote-Michael

[Copilot]

Pull request overview

This PR adds a custom ndo_eth_ioctl handler to the FEC (Freescale Ethernet Controller) driver to provide direct, side-effect-free MDIO register access from userspace via SIOCGMIIREG/SIOCSMIIREG, even when no phydev is attached (e.g., fixed-link to a DSA switch).

Changes:

• Introduces fec_enet_ioctl() to intercept SIOCGMIIREG and SIOCSMIIREG and execute reads/writes directly against fep->mii_bus.
• Keeps all other ioctl commands delegated to the existing phy_do_ioctl_running() path.
• Hooks the new handler into fec_netdev_ops via .ndo_eth_ioctl = fec_enet_ioctl.

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[AmeNote-Michael]

@Overdr0ne fyi I have tested with this added - does not solve APEC or PTP issues as noted to Audinate, not surprising. So will validate with Audinate if this is really needed.