netdev - Re: [PATCH net-next v4] net: dsa: realtek: rtl8365mb: add change

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Date:   Thu, 9 Mar 2023 23:55:46 -0300
From:   Luiz Angelo Daros de Luca <luizluca@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Simon Horman <simon.horman@...igine.com>, netdev@...r.kernel.org,
        linus.walleij@...aro.org, alsi@...g-olufsen.dk, andrew@...n.ch,
        vivien.didelot@...il.com, f.fainelli@...il.com, olteanv@...il.com,
        davem@...emloft.net, pabeni@...hat.com, robh+dt@...nel.org,
        krzk+dt@...nel.org, arinc.unal@...nc9.com,
        Alexander Duyck <alexanderduyck@...com>
Subject: Re: [PATCH net-next v4] net: dsa: realtek: rtl8365mb: add change_mtu

> Could I trouble you for v5 with some form of this explanation in the
> commit message?

Sure, here is a new proposed commit message:

The rtl8365mb was using a fixed MTU size of 1536, which was probably
inspired by the rtl8366rb's initial frame size. However, unlike that
family, the rtl8365mb family can specify the max frame size in bytes,
- rather than in fixed steps. The max packet size now defaults to
- VLAN_ETH_HLEN+ETH_DATA_LEN+ETH_FCS_LEN, which is 1522 bytes.
+ rather than in fixed steps.

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 specifications do not directly define the max supported
packet size, but it mentions a 16k limit. This driver will use the 0x3FFF
limit as it is used in the vendor API code. However, the switch sets the
max packet size to 16368 bytes (0x3FF0) after it resets.

change_mtu uses MTU size, or ethernet payload size, while the switch
works with frame size. The frame size is calculated considering the
ethernet header (14 bytes), a possible 802.1Q tag (4 bytes), the payload
size (MTU), and the Ethernet FCS (4 bytes). The CPU tag (8 bytes) is
consumed before the switch enforces the limit.

+ During setup, the driver will use the default 1500-byte MTU of DSA to set
+ the maximum frame size. The current sum will be
+ VLAN_ETH_HLEN+1500+ETH_FCS_LEN, which results in 1522 bytes.
+ Although it is lower than the previous initial value of 1536 bytes, the driver
+ will increase the frame size for a larger MTU. However, if something
+ requires more space without increasing the MTU, such as QinQ,
+ we would need to add the extra length to the rtl8365mb_port_change_mtu()
+ formula.
+
MTU was tested up to 2018 (with 802.1Q) as that is as far as mt7620
(where rtl8367s is stacked) can go. The register was manually
manipulated byte-by-byte to ensure the MTU to frame size conversion was
correct. For frames without 802.1Q tag, the frame size limit will be 4
bytes over the required size.

Let me know if I'm still not clear or missed some important topic.

Regards,

Luiz