Re: [PATCH iwl-next v4 3/3] igc: add support for forcing link speed without autonegotiation

[Simon Horman <horms@kernel.org>]

From: 'Simon Horman' <horms@kernel.org>

This is an AI-generated review of your patch. The human sending this
email has considered the AI review valid, or at least plausible.
Full review at: https://sashiko.dev
---
igc: add support for forcing link speed without autonegotiation

This commit allows users to force 10/100 Mb/s link speed and duplex via
ethtool when autonegotiation is disabled. Previously, the driver rejected
these requests.

> diff --git a/drivers/net/ethernet/intel/igc/igc_ethtool.c b/drivers/net/ethernet/intel/igc/igc_ethtool.c
> index cfcbf2fdad6e..9997ebbdf778 100644
> --- 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;

Does this permanently overwrite the user's flow control preference?

If half-duplex is requested, requested_mode is set to igc_fc_none. Because
requested_mode represents the persistent state for the user's flow control
preference, the user might permanently lose their configuration. If they later
switch back to full-duplex, requested_mode remains igc_fc_none, and flow
control stays disabled.

Would it be better to dynamically override hw->fc.current_mode to
igc_fc_none during link configuration instead of mutating requested_mode?

> +}
[ ... ]
> @@ -2071,14 +2126,20 @@ 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;
> +	}
> +

Should we also validate cmd->base.duplex here to prevent a fallback to
half-duplex?

If a user runs a command like "ethtool -s eth0 speed 100 autoneg off" while
the link is down, ethtool queries and preserves the current duplex value,
which is DUPLEX_UNKNOWN (0xff).

In igc_handle_autoneg_disabled(), the code checks if duplex == DUPLEX_FULL,
which evaluates to false, and forces the interface to 100 Mbps half-duplex.

Could we explicitly check that cmd->base.duplex is either DUPLEX_HALF or
DUPLEX_FULL and return -EINVAL otherwise?

>  	while (test_and_set_bit(__IGC_RESETTING, &adapter->state))
>  		usleep_range(1000, 2000);

This isn't a bug introduced by this patch, but does this ad-hoc
synchronization loop reimplement locking without fairness or lockdep support?

According to the Networking Subsystem Guidelines, using bit operations as a
substitute for real locks violates core kernel locking rules and introduces
potential races.

Could this be replaced with a proper concurrency mechanism such as a mutex?

>  
> -	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);
>  
>  	/* MDI-X => 2; MDI => 1; Auto => 3 */
>  	if (cmd->base.eth_tp_mdix_ctrl) {