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IEEE 802.1Q clause 12.29.1.1 "The queueMaxSDUTable structure and data types" and 8.6.8.4 "Enhancements for scheduled traffic" talk about the existence of a per traffic class limitation of maximum frame sizes, with a fallback on the port-based MTU. As far as I am able to understand, the 802.1Q Service Data Unit (SDU) represents the MAC Service Data Unit (MSDU, i.e. L2 payload), excluding any number of prepended VLAN headers which may be otherwise present in the MSDU. Therefore, the queueMaxSDU is directly comparable to the device MTU (1500 means L2 payload sizes are accepted, or frame sizes of 1518 octets, or 1522 plus one VLAN header). Drivers which offload this are directly responsible of translating into other units of measurement. To keep the fast path checks optimized, we keep 2 arrays in the qdisc, one for max_sdu translated into frame length (so that it's comparable to skb->len), and another for offloading and for dumping back to the user. Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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.