netdev - Re: [PATCH] net: dsa: mt7530: support setting MTU

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Date:   Thu, 29 Oct 2020 15:42:32 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     DENG Qingfang <dqfext@...il.com>
Cc:     netdev@...r.kernel.org, Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        René van Dorst <opensource@...rst.com>
Subject: Re: [PATCH] net: dsa: mt7530: support setting MTU

On Thu, 29 Oct 2020 02:12:21 +0800 DENG Qingfang wrote:
> MT7530/7531 has a global RX packet length register, which can be used
> to set MTU.
> 
> Signed-off-by: DENG Qingfang <dqfext@...il.com>

Please wrap your code at 80 chars.

> diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
> index de7692b763d8..7764c66a47c9 100644
> --- a/drivers/net/dsa/mt7530.c
> +++ b/drivers/net/dsa/mt7530.c
> @@ -1021,6 +1021,40 @@ mt7530_port_disable(struct dsa_switch *ds, int port)
>  	mutex_unlock(&priv->reg_mutex);
>  }
>  
> +static int
> +mt7530_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
> +{
> +	struct mt7530_priv *priv = ds->priv;
> +	int length;
> +
> +	/* When a new MTU is set, DSA always set the CPU port's MTU to the largest MTU
> +	 * of the slave ports. Because the switch only has a global RX length register,
> +	 * only allowing CPU port here is enough.
> +	 */
> +	if (!dsa_is_cpu_port(ds, port))
> +		return 0;
> +
> +	/* RX length also includes Ethernet header, MTK tag, and FCS length */
> +	length = new_mtu + ETH_HLEN + MTK_HDR_LEN + ETH_FCS_LEN;
> +	if (length <= 1522)
> +		mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_PKT_LEN_MASK, MAX_RX_PKT_LEN_1522);
> +	else if (length <= 1536)
> +		mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_PKT_LEN_MASK, MAX_RX_PKT_LEN_1536);
> +	else if (length <= 1552)
> +		mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_PKT_LEN_MASK, MAX_RX_PKT_LEN_1552);
> +	else
> +		mt7530_rmw(priv, MT7530_GMACCR, MAX_RX_JUMBO_MASK | MAX_RX_PKT_LEN_MASK,
> +			MAX_RX_JUMBO(DIV_ROUND_UP(length, 1024)) | MAX_RX_PKT_LEN_JUMBO);

this line should start under priv, so it aligns to the opening
parenthesis.

Besides, don't you need to reset the JUMBO bit when going from jumbo to
non-jumbo? The mask should always include jumbo.

I assume you're duplicating the mt7530_rmw() for the benefit of the
constant validation, but it seems to be counterproductive here.

> +	return 0;
> +}