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

[Simon Horman <horms@kernel.org>]

On Mon, Apr 20, 2026 at 11:20:07AM +0800, Abdul Rahim, Faizal wrote:
> 
> 
> On 19/4/2026 12:48 am, Simon Horman wrote:
> > 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 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?
> > 
> 
> You're right, thanks for pointing that out.
> 
> That said, it feels simpler to address it with [1]:
> if (duplex != DUPLEX_FULL)
>     adapter->hw.fc.requested_mode = igc_fc_none;
> 
> Rather than [2]:
>  if (mac->forced_speed_duplex == IGC_FORCED_10H ||
>         mac->forced_speed_duplex == IGC_FORCED_100H)
>         adapter->hw.fc.requested_mode = igc_fc_none;
> 
> Are you okay with [1] ?

Yes, [1] sounds sensible to me.

> 
> > [ ... ]
> > 
> > > @@ -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?
> 
> It looks like a worthwhile cleanup. However, it likely doesn’t belong in
> this series, since the synchronization pattern predates these patches and is
> used throughout the igc driver (set_ringparam, set_pauseparam, set_channels,
> etc.). We could address it in different patch series and align the other
> code paths at the same time ?

Yes, agreed.