[PATCH] net: macb: add TX stall timeout callback to recover from lost TSTART write

[Théo Lebrun]

Hello Andrea,

On Fri Jun 12, 2026 at 11:01 AM CEST, Andrea della Porta wrote:
> From: Lukasz Raczylo <lukasz at raczylo.com>
>
> The MACB found in the Raspberry Pi RP1 suffers from sporadic stalls on
> the TX queue.
> While the exact root cause is not yet fully understood, it is likely
> related to a hardware issue where a TSTART write to the NCR register
> is missed, preventing the transmission from being kicked off.
>
> Implement a timeout callback to handle TX queue stalls, triggering the
> existing restart mechanism to recover.
>
> Link: https://lore.kernel.org/all/20260514215459.36109-1-lukasz@raczylo.com/
> Fixes: dc110d1b23564 ("net: cadence: macb: Add support for Raspberry Pi RP1 ethernet controller")
> Signed-off-by: Lukasz Raczylo <lukasz at raczylo.com>
> Co-developed-by: Steffen Jaeckel <sjaeckel at suse.de>
> Signed-off-by: Steffen Jaeckel <sjaeckel at suse.de>
> Co-developed-by: Andrea della Porta <andrea.porta at suse.com>
> Signed-off-by: Andrea della Porta <andrea.porta at suse.com>
> ---
>  drivers/net/ethernet/cadence/macb_main.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
> index a12aa21244e83..615da65d5d68d 100644
> --- a/drivers/net/ethernet/cadence/macb_main.c
> +++ b/drivers/net/ethernet/cadence/macb_main.c
> @@ -4522,6 +4522,16 @@ static int macb_setup_tc(struct net_device *dev, enum tc_setup_type type,
>  	}
>  }
>  
> +static void macb_tx_timeout(struct net_device *dev, unsigned int q)
> +{
> +	struct macb *bp = netdev_priv(dev);
> +
> +	if (net_ratelimit())
> +		netdev_err(dev, "TX stall detected, re-kicking TSTART\n");

Is this standard? It looks odd.

> +	dev->stats.tx_errors++;

I am surprised by this. `tx_errors` would ideally be one per packet that
didn't get sent. Here we increment it once per queue that stalled.

I have a series to address stats issue (and use netdev_stat_ops API).
It is a follow-up to this:
https://lore.kernel.org/netdev/20260428-macb-drop-tx-v2-0-647f5199d8df@bootlin.com/

Also this is per-device shared data and we access it without
synchronisation.

Let's drop this increment.

> +	macb_tx_restart(&bp->queues[q]);
> +}

Regards,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com