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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. DSA calls change_mtu for the CPU port once the max MTU value among the ports changes. As the max frame 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 frame 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 frame 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. There is a jumbo register, enabled by default at 6k frame size. However, the jumbo settings do 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@gmail.com> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com> Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.