Merge branch 'add-tc-taprio-support-for-queuemaxsdu'

Merge branch 'add-tc-taprio-support-for-queuemaxsdu'

Vladimir Oltean says:

====================
Add tc-taprio support for queueMaxSDU

The tc-taprio offload mode supported by the Felix DSA driver has
limitations surrounding its guard bands.

The initial discussion was at:
https://lore.kernel.org/netdev/c7618025da6723418c56a54fe4683bd7@walle.cc/

with the latest status being that we now have a vsc9959_tas_guard_bands_update()
method which makes a best-guess attempt at how much useful space to
reserve for packet scheduling in a taprio interval, and how much to
reserve for guard bands.

IEEE 802.1Q actually does offer a tunable variable (queueMaxSDU) which
can determine the max MTU supported per traffic class. In turn we can
determine the size we need for the guard bands, depending on the
queueMaxSDU. This way we can make the guard band of small taprio
intervals smaller than one full MTU worth of transmission time, if we
know that said traffic class will transport only smaller packets.

As discussed with Gerhard Engleder, the queueMaxSDU may also be useful
in limiting the latency on an endpoint, if some of the TX queues are
outside of the control of the Linux driver.
https://patchwork.kernel.org/project/netdevbpf/patch/20220914153303.1792444-11-vladimir.oltean@nxp.com/

Allow input of queueMaxSDU through netlink into tc-taprio, offload it to
the hardware I have access to (LS1028A), and (implicitly) deny
non-default values to everyone else. Kurt Kanzenbach has also kindly
tested and shared a patch to offload this to hellcreek.

v3 at:
https://patchwork.kernel.org/project/netdevbpf/cover/20220927234746.1823648-1-vladimir.oltean@nxp.com/
v2 at:
https://patchwork.kernel.org/project/netdevbpf/list/?series=679954&state=*
v1 at:
https://patchwork.kernel.org/project/netdevbpf/cover/20220914153303.1792444-1-vladimir.oltean@nxp.com/
====================

Link: https://lore.kernel.org/r/20220928095204.2093716-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>