Message-ID: <20230209185147.lsyrxzjlz65mohy3@bang-olufsen.dk>
Date:   Thu, 9 Feb 2023 18:51:48 +0000
From:   Alvin Šipraga <ALSI@...g-olufsen.dk>
To:     Luiz Angelo Daros de Luca <luizluca@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "vivien.didelot@...il.com" <vivien.didelot@...il.com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "olteanv@...il.com" <olteanv@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzk+dt@...nel.org" <krzk+dt@...nel.org>,
        "arinc.unal@...nc9.com" <arinc.unal@...nc9.com>
Subject: Re: [PATCH net-next v2] net: dsa: realtek: rtl8365mb: add change_mtu

On Tue, Feb 07, 2023 at 02:15:58PM -0300, Luiz Angelo Daros de Luca wrote:
> rtl8365mb was using a fixed MTU size of 1536, probably inspired by
> rtl8366rb initial packet size. Different from that family, rtl8365mb
> family can specify the max packet size in bytes and not in fixed steps.
> Now it defaults to VLAN_ETH_HLEN+ETH_DATA_LEN+ETH_FCS_LEN (1522 bytes).
> 
> DSA calls change_mtu for the CPU port once the max mtu value among the
> ports changes. As the max packet size is defined globally, the switch
> is configured only when the call affects the CPU port.
> 
> The available specs do not directly define the max supported packet
> size, but it mentions a 16k limit. However, the switch sets the max
> packet size to 16368 bytes (0x3FF0) after it resets. That value was
> assumed as the maximum supported packet size.
> 
> MTU was tested up to 2018 (with 802.1Q) as that is as far as mt7620
> (where rtl8367s is stacked) can go.

Thinking about it, I'm sure why you would test this with 802.1Q
specifically. Maybe you can elaborate on how you tested this?

> 
> There is a jumbo register, enabled by default at 6k packet size.
> However, the jumbo settings does not seem to limit nor expand the
> maximum tested MTU (2018), even when jumbo is disabled. More tests are
> needed with a device that can handle larger frames.
> 
> Signed-off-by: Luiz Angelo Daros de Luca <luizluca@...il.com>
> ---
> 
> v1->v2:
> - dropped jumbo code as it was not changing the behavior (up to 2k MTU)
> - fixed typos
> - fixed code alignment
> - renamed rtl8365mb_(change|max)_mtu to rtl8365mb_port_(change|max)_mtu
> 
>  drivers/net/dsa/realtek/rtl8365mb.c | 43 ++++++++++++++++++++++++++---
>  1 file changed, 39 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/dsa/realtek/rtl8365mb.c b/drivers/net/dsa/realtek/rtl8365mb.c
> index da31d8b839ac..c3e0a5b55738 100644
> --- a/drivers/net/dsa/realtek/rtl8365mb.c
> +++ b/drivers/net/dsa/realtek/rtl8365mb.c
> @@ -98,6 +98,7 @@
>  #include <linux/of_irq.h>
>  #include <linux/regmap.h>
>  #include <linux/if_bridge.h>
> +#include <linux/if_vlan.h>
>  
>  #include "realtek.h"
>  
> @@ -267,6 +268,8 @@
>  /* Maximum packet length register */
>  #define RTL8365MB_CFG0_MAX_LEN_REG	0x088C
>  #define   RTL8365MB_CFG0_MAX_LEN_MASK	0x3FFF
> +/* Not sure but it is the default value after reset */
> +#define RTL8365MB_CFG0_MAX_LEN_MAX	0x3FF0

Again, the max is 0x3FFF per the automatically generated register map
from the vendor driver. Unless you have evidence to the contrary. Please
fix this.

>  
>  /* Port learning limit registers */
>  #define RTL8365MB_LUT_PORT_LEARN_LIMIT_BASE		0x0A20
> @@ -1135,6 +1138,36 @@ static void rtl8365mb_phylink_mac_link_up(struct dsa_switch *ds, int port,
>  	}
>  }
>  
> +static int rtl8365mb_port_change_mtu(struct dsa_switch *ds, int port,
> +				     int new_mtu)
> +{
> +	struct realtek_priv *priv = ds->priv;
> +	int frame_size;
> +
> +	/* When a new MTU is set, DSA always sets 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.
> +	 */
> +

Spurious newline.

> +	if (!dsa_is_cpu_port(ds, port))
> +		return 0;
> +
> +	frame_size = new_mtu + VLAN_ETH_HLEN + ETH_FCS_LEN;

I'm still not convinced that this is correct. dsa_master_setup() sets
the master MTU to ETH_DATA_LEN + dsa_tag_protocol_overhead(tag_ops) by
default. So even if you wanted to make space for 802.1Q-tagged packets
whose payload was 1500 bytes, they wouldn't make it past the master, as
they are 4 octets too big for its MTU. Or what am I missing?

On the other hand, since we are talking total frame size here, shouldn't
you should also compensate for the CPU tag (8 bytes)? Did you check
this? For example, maybe the switch applies the frame size check after
popping the CPU tag and that's why you don't add it?

I think the comment in dsa.h is helpful:

	/*
	 * MTU change functionality. Switches can also adjust their MRU through
	 * this method. By MTU, one understands the SDU (L2 payload) length.
	 * If the switch needs to account for the DSA tag on the CPU port, this
	 * method needs to do so privately.
	 */
	int	(*port_change_mtu)(struct dsa_switch *ds, int port,
				   int new_mtu);
	int	(*port_max_mtu)(struct dsa_switch *ds, int port);


My bid would be:

	frame_size = new_mtu + 8 + ETH_HLEN + ETH_FCS_LEN;

where the 8 is for the CPU tag.

> +
> +	dev_dbg(priv->dev, "changing mtu to %d (frame size: %d)\n",
> +		new_mtu, frame_size);
> +
> +	return regmap_update_bits(priv->map, RTL8365MB_CFG0_MAX_LEN_REG,
> +				  RTL8365MB_CFG0_MAX_LEN_MASK,
> +				  FIELD_PREP(RTL8365MB_CFG0_MAX_LEN_MASK,
> +					     frame_size));
> +}
> +
> +static int rtl8365mb_port_max_mtu(struct dsa_switch *ds, int port)
> +{
> +	return RTL8365MB_CFG0_MAX_LEN_MAX - VLAN_ETH_HLEN - ETH_FCS_LEN;
> +}
> +
>  static void rtl8365mb_port_stp_state_set(struct dsa_switch *ds, int port,
>  					 u8 state)
>  {
> @@ -1980,10 +2013,8 @@ static int rtl8365mb_setup(struct dsa_switch *ds)
>  		p->index = i;
>  	}
>  
> -	/* Set maximum packet length to 1536 bytes */
> -	ret = regmap_update_bits(priv->map, RTL8365MB_CFG0_MAX_LEN_REG,
> -				 RTL8365MB_CFG0_MAX_LEN_MASK,
> -				 FIELD_PREP(RTL8365MB_CFG0_MAX_LEN_MASK, 1536));
> +	/* Set packet length from 16368 to 1500+14+4+4=1522 */

This comment is not very helpful IMO...

> +	ret = rtl8365mb_port_change_mtu(ds, cpu->trap_port, ETH_DATA_LEN);
>  	if (ret)
>  		goto out_teardown_irq;
>  
> @@ -2103,6 +2134,8 @@ static const struct dsa_switch_ops rtl8365mb_switch_ops_smi = {
>  	.get_eth_mac_stats = rtl8365mb_get_mac_stats,
>  	.get_eth_ctrl_stats = rtl8365mb_get_ctrl_stats,
>  	.get_stats64 = rtl8365mb_get_stats64,
> +	.port_change_mtu = rtl8365mb_port_change_mtu,
> +	.port_max_mtu = rtl8365mb_port_max_mtu,
>  };
>  
>  static const struct dsa_switch_ops rtl8365mb_switch_ops_mdio = {
> @@ -2124,6 +2157,8 @@ static const struct dsa_switch_ops rtl8365mb_switch_ops_mdio = {
>  	.get_eth_mac_stats = rtl8365mb_get_mac_stats,
>  	.get_eth_ctrl_stats = rtl8365mb_get_ctrl_stats,
>  	.get_stats64 = rtl8365mb_get_stats64,
> +	.port_change_mtu = rtl8365mb_port_change_mtu,
> +	.port_max_mtu = rtl8365mb_port_max_mtu,
>  };
>  
>  static const struct realtek_ops rtl8365mb_ops = {
> -- 
> 2.39.1
>

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