Re: [PATCH iwl-net v2 3/6] ixgbe: call ixgbe_setup_fc() before fc_enable() after NVM update

[Simon Horman <horms@kernel.org>]

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

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---
ixgbe: call ixgbe_setup_fc() before fc_enable() after NVM update

This commit adds a call to hw->mac.ops.setup_fc() before fc_enable() in the
watchdog task when the link comes up, aiming to prevent MDD events after
an NVM update by reprogramming flow-control autonegotiation.

> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -8031,7 +8031,14 @@ static void ixgbe_watchdog_update_link(struct ixgbe_adapter *adapter)
>  		pfc_en |= !!(adapter->ixgbe_ieee_pfc->pfc_en);
>  
>  	if (link_up && !((adapter->flags & IXGBE_FLAG_DCB_ENABLED) && pfc_en)) {
> -		hw->mac.ops.fc_enable(hw);
> +		/* Re-program flow-control autoneg before applying the result.
> +		 * If setup_fc() fails its output is invalid; skip fc_enable()
> +		 * to avoid committing stale capability bits that trigger MDD.
> +		 */
> +		if (hw->mac.ops.setup_fc && hw->mac.ops.setup_fc(hw))

Will calling setup_fc() unconditionally from the link-up watchdog cause an
infinite link flapping loop on 82599 backplane interfaces?

For backplane interfaces like KX/KX4/KR, setup_fc() resolves to
ixgbe_setup_fc_generic(), which then calls hw->mac.ops.prot_autoc_write().

On 82599 hardware, this calls prot_autoc_write_82599(), which unconditionally
executes ixgbe_reset_pipeline_82599().

Since ixgbe_reset_pipeline_82599() toggles the IXGBE_AUTOC_AN_RESTART bit,
would this drop the link immediately to restart Auto-Negotiation?

If the link drops, it would trigger a link-down LSC interrupt. Once
Auto-Negotiation completes, the link would come back up, triggering another
link-up LSC interrupt. This would cause ixgbe_watchdog_update_link() to run
again, repeating the cycle.

> +			e_warn(drv, "setup_fc failed, skipping fc_enable\n");
> +		else
> +			hw->mac.ops.fc_enable(hw);
>  		ixgbe_set_rx_drop_en(adapter);
>  	}