Re: [PATCH net-next v1 01/21] net: usb: lan78xx: Remove LAN8835 PHY fixup

On Tue, Dec 03, 2024 at 08:21:34AM +0100, Oleksij Rempel wrote:

Remove the PHY fixup for the LAN8835 PHY in the lan78xx driver due to
the following reasons:

- There is no publicly available information about the LAN8835 PHY.
  However, it appears to be the integrated PHY used in the LAN7800 and
  LAN7850 USB Ethernet controllers. These PHYs use the GMII interface,
  not RGMII as configured by the fixup.

- The correct driver for handling the LAN8835 PHY functionality is the
  Microchip PHY driver (`drivers/net/phy/microchip.c`), which properly
  supports these integrated PHYs.

- The PHY ID `0x0007C130` is actually used by the LAN8742A PHY, which
  only supports RMII. This interface is incompatible with the LAN78xx
  MAC, as the LAN7801 (the only LAN78xx version without an integrated
  PHY) supports only RGMII.

- The mask applied for this fixup is overly broad, inadvertently
  covering both Microchip LAN88xx PHYs and unrelated SMSC LAN8742A PHYs,
  leading to potential conflicts with other devices.

- Testing has shown that removing this fixup for LAN7800 and LAN7850
  does not result in any noticeable difference in functionality, as the
  Microchip PHY driver (`drivers/net/phy/microchip.c`) handles all
  necessary configurations for these integrated PHYs.

- Registering this fixup globally (not limited to USB devices) risks
  conflicts by unintentionally modifying other interfaces whenever a
  LAN7801 adapter is connected to the system.

Note that both LAN7800 and LAN7850 USB Ethernet controllers use an
integrated PHY with the ID `0x0007C132`. Additionally, the LAN7515, a
specialized part for Raspberry Pi, includes an integrated LAN7800 USB
Ethernet controller and USB hub in a multifunctional chip design, and it
also uses the same PHY ID (`0x0007C132`).

I had a long frustrating discussion about adding yet more such fixups
a while ago with somebody how did not understand the implications of
adding another one. It is good to see this one being removed, with a
good explanation why.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew