Re: [RESEND net-next PATCH 3/3] net: dsa: qca8k: reset cpu port on MTU change

[Jakub Kicinski <kuba@kernel.org>]

On Sat, 18 Jun 2022 09:26:50 +0200 Christian Marangi wrote:
> It was discovered that the Documentation lacks of a fundamental detail
> on how to correctly change the MAX_FRAME_SIZE of the switch.
> 
> In fact if the MAX_FRAME_SIZE is changed while the cpu port is on, the
> switch panics and cease to send any packet. This cause the mgmt ethernet
> system to not receive any packet (the slow fallback still works) and
> makes the device not reachable. To recover from this a switch reset is
> required.
> 
> To correctly handle this, turn off the cpu ports before changing the
> MAX_FRAME_SIZE and turn on again after the value is applied.
> 
> Fixes: f58d2598cf70 ("net: dsa: qca8k: implement the port MTU callbacks")
> Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
 
It reads like this patch should be backported to 5.10 and 5.15 stable
branches. While patches 1 and 2 are cleanups. In which case you should
reports just patch 3 against net/master first, we'll send it to Linus at
the end of the week and then you can send the cleanups on top for -next.

One extra question below.

> diff --git a/drivers/net/dsa/qca8k.c b/drivers/net/dsa/qca8k.c
> index eaaf80f96fa9..0b92b9d5954a 100644
> --- a/drivers/net/dsa/qca8k.c
> +++ b/drivers/net/dsa/qca8k.c
> @@ -2334,6 +2334,7 @@ static int
>  qca8k_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
>  {
>  	struct qca8k_priv *priv = ds->priv;
> +	int ret;
>  
>  	/* We have only have a general MTU setting.
>  	 * DSA always set the CPU port's MTU to the largest MTU of the slave
> @@ -2344,10 +2345,29 @@ qca8k_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
>  	if (!dsa_is_cpu_port(ds, port))
>  		return 0;
>  
> +	/* To change the MAX_FRAME_SIZE the cpu ports must be off or
> +	 * the switch panics.
> +	 * Turn off both cpu ports before applying the new value to prevent
> +	 * this.
> +	 */
> +	if (priv->port_enabled_map & BIT(0))
> +		qca8k_port_set_status(priv, 0, 0);
> +
> +	if (priv->port_enabled_map & BIT(6))
> +		qca8k_port_set_status(priv, 6, 0);
> +
>  	/* Include L2 header / FCS length */
> -	return regmap_update_bits(priv->regmap, QCA8K_MAX_FRAME_SIZE_REG,
> -				  QCA8K_MAX_FRAME_SIZE_MASK,
> -				  new_mtu + ETH_HLEN + ETH_FCS_LEN);
> +	ret = regmap_update_bits(priv->regmap, QCA8K_MAX_FRAME_SIZE_REG,

Why care about the return code of this regmap access but not the ones
inside the *port_set_status() calls?

> +				 QCA8K_MAX_FRAME_SIZE_MASK,
> +				 new_mtu + ETH_HLEN + ETH_FCS_LEN);
> +
> +	if (priv->port_enabled_map & BIT(0))
> +		qca8k_port_set_status(priv, 0, 1);
> +
> +	if (priv->port_enabled_map & BIT(6))
> +		qca8k_port_set_status(priv, 6, 1);
> +
> +	return ret;
>  }
>  
>  static int