Re: [PATCH iwl-next v3 3/3] igc: add support for forcing link speed without autonegotiation

[Simon Horman <horms@kernel.org>]

On Wed, Apr 22, 2026 at 11:57:01PM +0800, KhaiWenTan wrote:
> From: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
> 
> Allow users to force 10/100 Mb/s link speed and duplex via ethtool
> when autonegotiation is disabled. Previously, the driver rejected
> these requests with "Force mode currently not supported.".
> 
> Forcing at 1000 Mb/s and 2500 Mb/s is not supported.
> 
> Reviewed-by: Looi, Hong Aun <hong.aun.looi@intel.com>
> Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
> Signed-off-by: KhaiWenTan <khai.wen.tan@linux.intel.com>

...

> diff --git a/drivers/net/ethernet/intel/igc/igc_ethtool.c b/drivers/net/ethernet/intel/igc/igc_ethtool.c

...

> @@ -2000,6 +2013,41 @@ static int igc_ethtool_get_link_ksettings(struct net_device *netdev,
>  	return 0;
>  }
>  
> +/**
> + * igc_handle_autoneg_disabled - Configure forced speed/duplex settings
> + * @adapter: private driver structure
> + * @speed: requested speed (must be SPEED_10 or SPEED_100)
> + * @duplex: requested duplex
> + *
> + * Records forced speed/duplex when autoneg is disabled.
> + * Caller must validate speed before calling this function.
> + */
> +static void igc_handle_autoneg_disabled(struct igc_adapter *adapter, u32 speed,
> +					u8 duplex)
> +{
> +	struct igc_mac_info *mac = &adapter->hw.mac;
> +
> +	switch (speed) {
> +	case SPEED_10:
> +		mac->forced_speed_duplex = (duplex == DUPLEX_FULL) ?
> +			IGC_FORCED_10F : IGC_FORCED_10H;
> +		break;
> +	case SPEED_100:
> +		mac->forced_speed_duplex = (duplex == DUPLEX_FULL) ?
> +			IGC_FORCED_100F : IGC_FORCED_100H;
> +		break;
> +	default:
> +		WARN_ONCE(1, "Unsupported speed %u\n", speed);
> +		return;
> +	}
> +
> +	mac->autoneg_enabled = false;
> +
> +	/* Half-duplex cannot support flow control per IEEE 802.3 */
> +	if (duplex != DUPLEX_FULL)
> +		adapter->hw.fc.requested_mode = igc_fc_none;
> +}
> +
>  /**
>   * igc_handle_autoneg_enabled - Configure autonegotiation advertisement
>   * @adapter: private driver structure
> @@ -2038,6 +2086,7 @@ static void igc_handle_autoneg_enabled(struct igc_adapter *adapter,
>  						  10baseT_Half))
>  		advertised |= ADVERTISE_10_HALF;
>  
> +	hw->mac.autoneg_enabled = true;
>  	hw->phy.autoneg_advertised = advertised;
>  	if (adapter->fc_autoneg)
>  		hw->fc.requested_mode = igc_fc_default;
> @@ -2071,14 +2120,20 @@ igc_ethtool_set_link_ksettings(struct net_device *netdev,
>  		}
>  	}
>  
> +	if (cmd->base.autoneg == AUTONEG_DISABLE &&
> +	    cmd->base.speed != SPEED_10 && cmd->base.speed != SPEED_100) {
> +		netdev_info(dev, "Unsupported speed for forced link\n");
> +		return -EINVAL;
> +	}

The condition above verifies speed only if autoneg is AUTONEG_DISABLE.

> +
>  	while (test_and_set_bit(__IGC_RESETTING, &adapter->state))
>  		usleep_range(1000, 2000);
>  
> -	if (cmd->base.autoneg == AUTONEG_ENABLE) {
> +	if (cmd->base.autoneg == AUTONEG_ENABLE)
>  		igc_handle_autoneg_enabled(adapter, cmd);
> -	} else {
> -		netdev_info(dev, "Force mode currently not supported\n");
> -	}
> +	else
> +		igc_handle_autoneg_disabled(adapter, cmd->base.speed,
> +					    cmd->base.duplex);

But here igc_handle_autoneg_disabled, which relies on speed having been
verified, is called if autoneg is not AUTONEG_ENABLE.

If autoneg is AUTONEG_DISABLE here, then all is good.
But if it is neither AUTONEG_DISABLE nor AUTONEG_ENABLE then we
are in trouble.

I suggest verifying autoneg is either AUTONEG_ENABLE or AUTONEG_DISABLE
earlier in this function.

The above is based on my analysis of review AI generated review from Sashiko.
I believe that addressing autoneg verification address the review by
Sashiko that is relevant to the progress of this patch.

...