This switch implements Hold/Release in a strange way, with no control
from the user as required by IEEE 802.1Q-2018 through Set-And-Hold-MAC
and Set-And-Release-MAC, but rather, it emits HOLD requests implicitly
based on the schedule.

Namely, when the gate of a preemptible TC is about to close (actually
QSYS::PREEMPTION_CFG.HOLD_ADVANCE octet times in advance of this event),
the QSYS seems to emit a HOLD request pulse towards the MAC which
preempts the currently transmitted packet, and further packets are held
back in the queue system.

This allows large frames to be squeezed through small time slots,
because HOLD requests initiated by the gate events result in the frame
being segmented in multiple fragments, the bit time of which is equal to
the size of the time slot.

It has been reported that the vsc9959_tas_guard_bands_update() logic
breaks this, because it doesn't take preemptible TCs into account, and
enables oversized frame dropping when the time slot doesn't allow a full
MTU to be sent, but it does allow 2*minFragSize to be sent (128B).
Packets larger than 128B are dropped instead of being sent in multiple
fragments.

Confusingly, the manual says:

| For guard band, SDU calculation of a traffic class of a port, if
| preemption is enabled (through 'QSYS::PREEMPTION_CFG.P_QUEUES') then
| QSYS::PREEMPTION_CFG.HOLD_ADVANCE is used, otherwise
| QSYS::QMAXSDU_CFG_*.QMAXSDU_* is used.

but this only refers to the static guard band durations, and the
QMAXSDU_CFG_* registers have dual purpose - the other being oversized
frame dropping, which takes place irrespective of whether frames are
preemptible or express.

So, to fix the problem, we need to call vsc9959_tas_guard_bands_update()
from ocelot_port_update_active_preemptible_tcs(), and modify the guard
band logic to consider a different (lower) oversize limit for
preemptible traffic classes.

Fixes: 403ffc2c ("net: mscc: ocelot: add support for preemptible traffic classes")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-ID: <20230705104422.49025-4-vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>