Re: [PATCH net-next v1 02/21] net: usb: lan78xx: Remove KSZ9031 PHY fixup

[Andrew Lunn]

On Tue, Dec 03, 2024 at 08:21:35AM +0100, Oleksij Rempel wrote:
> Remove the KSZ9031RNX PHY fixup from the lan78xx driver. The fixup applied
> specific RGMII pad skew configurations globally, but these settings violate the
> RGMII specification and cause more harm than benefit.
> 
> Key issues with the fixup:
> 1. **Non-Compliant Timing**: The fixup's delay settings fall outside the RGMII
>    specification requirements of 1.5 ns to 2.0 ns:
>    - RX Path: Total delay of **2.16 ns** (PHY internal delay of 1.2 ns + 0.96
>      ns skew).
>    - TX Path: Total delay of **0.96 ns**, significantly below the RGMII minimum
>      of 1.5 ns.
> 
> 2. **Redundant or Incorrect Configurations**:
>    - The RGMII skew registers written by the fixup do not meaningfully alter
>      the PHY's default behavior and fail to account for its internal delays.
>    - The TX_DATA pad skew was not configured, relying on power-on defaults
>      that are insufficient for RGMII compliance.
> 
> 3. **Micrel Driver Support**: By setting `PHY_INTERFACE_MODE_RGMII_ID`, the
>    Micrel driver can calculate and assign appropriate skew values for the
>    KSZ9031 PHY.  This ensures better timing configurations without relying on
>    external fixups.
> 
> 4. **System Interference**: The fixup applied globally, reconfiguring all
>    KSZ9031 PHYs in the system, even those unrelated to the LAN78xx adapter.
>    This could lead to unintended and harmful behavior on unrelated interfaces.
> 
> While the fixup is removed, a better mechanism is still needed to dynamically
> determine the optimal combination of PHY and MAC delays to fully meet RGMII
> requirements without relying on Device Tree or global fixups. This would allow
> for robust operation across different hardware configurations.
> 
> The Micrel driver is capable of using the interface mode value to calculate and
> apply better skew values, providing a configuration much closer to the RGMII
> specification than the fixup. Removing the fixup ensures better default
> behavior and prevents harm to other system interfaces.

PHY_INTERFACE_MODE_RGMII_ID should be good enough for most
hardware. It seems like USB dongle developers don't really understand
phylib.

Reviewed-by: Andrew Lunn <andrew@xxxxxxx>

    Andrew