Re: [PATCH iwl-next v2 3/3] igc: add support for forcing link speed without autonegotiation

[Simon Horman <horms@kernel.org>]

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

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---
igc: add support for forcing link speed without autonegotiation

This commit adds support to allow users to force 10/100 Mb/s link speed
and duplex via ethtool when autonegotiation is disabled.

> diff --git a/drivers/net/ethernet/intel/igc/igc_ethtool.c b/drivers/net/ethernet/intel/igc/igc_ethtool.c
> index cfcbf2fdad6ea..5bd37d1be1688 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_HALF)
> +		adapter->hw.fc.requested_mode = igc_fc_none;
> +}

If userspace issues an ethtool command without specifying duplex, the duplex
value can be DUPLEX_UNKNOWN. In this case, forced_speed_duplex correctly
defaults to half-duplex since duplex == DUPLEX_FULL evaluates to false. 

However, since DUPLEX_UNKNOWN is not equal to DUPLEX_HALF, the check above
fails, and the requested flow control mode remains unchanged. Could this leave
flow control enabled on a forced half-duplex link, violating IEEE 802.3?
Should this check be based on the resolved mac->forced_speed_duplex state
instead?

[ ... ]

> @@ -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;
> +	}
> +
>  	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 using bit flags circumvent proper locking guarantees? The
networking subsystem guidelines forbid using bit flags as ad-hoc locks. 

Using a bit flag in a polling loop bypasses lockdep analysis, fairness, and
strict memory ordering. Could this be replaced with a real synchronization
mechanism like a mutex?