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

[Abdul Rahim, Faizal]

On 30/4/2026 9:50 pm, Simon Horman wrote:
> From: 'Simon Horman' <horms@xxxxxxxxxx>
> 
> 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?
> 

Yes you're right, thanks, will update.

>> +}
> [ ... ]
>> @@ -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?

Will update.

> 
>>  	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?
> 

Hi Simon, you've raised this issue in v2, and after discussion, you've
agreed that this change doesn't belong in this patch series.

Not sure if I missed anything?

>>  
>> -	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) {
>