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

From: Simon Horman <[email-masked]>
Date: Thu, 5 Dec 2024 17:12:03 +0000

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.
> 
> Signed-off-by: Oleksij Rempel <[email-masked]>
> ---
>  drivers/net/usb/lan78xx.c | 38 +++++---------------------------------
>  1 file changed, 5 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/net/usb/lan78xx.c b/drivers/net/usb/lan78xx.c

...

> @@ -2283,14 +2263,11 @@ static struct phy_device *lan7801_phy_init(struct lan78xx_net *dev)
>  			netdev_err(dev->net, "no PHY driver found\n");
>  			return NULL;
>  		}
> -		dev->interface = PHY_INTERFACE_MODE_RGMII;
> -		/* external PHY fixup for KSZ9031RNX */
> -		ret = phy_register_fixup_for_uid(PHY_KSZ9031RNX, 0xfffffff0,
> -						 ksz9031rnx_fixup);
> -		if (ret < 0) {
> -			netdev_err(dev->net, "Failed to register fixup for PHY_KSZ9031RNX\n");
> -			return NULL;
> -		}
> +		dev->interface = PHY_INTERFACE_MODE_RGMII_ID;
> +		/* The PHY driver is responsible to configure proper RGMII
> +		 * interface delays. Disable RGMII delays on MAC side.
> +		 */
> +		lan78xx_write_reg(dev, MAC_RGMII_ID, 0);
>  
>  		phydev->is_internal = false;
>  	}

nit: With this change ret is now set but otherwise unused in this function.

     I would suggest a new patch, prior to this one, that drops the
     other places where it is set. They seem to have no effect.
     And then remove ret from this function in this patch.

> @@ -2349,9 +2326,6 @@ static int lan78xx_phy_init(struct lan78xx_net *dev)
>  			if (phy_is_pseudo_fixed_link(phydev)) {
>  				fixed_phy_unregister(phydev);
>  				phy_device_free(phydev);
> -			} else {
> -				phy_unregister_fixup_for_uid(PHY_KSZ9031RNX,
> -							     0xfffffff0);
>  			}
>  		}
>  		return -EIO;

...