net: dsa: felix: tc-taprio intervals smaller than MTU should send at least one packet

The blamed commit broke tc-taprio schedules such as this one:

tc qdisc replace dev $swp1 root taprio \
        num_tc 8 \
        map 0 1 2 3 4 5 6 7 \
        queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \
        base-time 0 \
        sched-entry S 0x7f 990000 \
        sched-entry S 0x80  10000 \
        flags 0x2

because the gate entry for TC 7 (S 0x80 10000 ns) now has a static guard
band added earlier than its 'gate close' event, such that packet
overruns won't occur in the worst case of the largest packet possible.

Since guard bands are statically determined based on the per-tc
QSYS_QMAXSDU_CFG_* with a fallback on the port-based QSYS_PORT_MAX_SDU,
we need to discuss what happens with TC 7 depending on kernel version,
since the driver, prior to commit 55a515b1 ("net: dsa: felix: drop
oversized frames with tc-taprio instead of hanging the port"), did not
touch QSYS_QMAXSDU_CFG_*, and therefore relied on QSYS_PORT_MAX_SDU.

1 (before vsc9959_tas_guard_bands_update): QSYS_PORT_MAX_SDU defaults to
  1518, and at gigabit this introduces a static guard band (independent
  of packet sizes) of 12144 ns, plus QSYS::HSCH_MISC_CFG.FRM_ADJ (bit
  time of 20 octets => 160 ns). But this is larger than the time window
  itself, of 10000 ns. So, the queue system never considers a frame with
  TC 7 as eligible for transmission, since the gate practically never
  opens, and these frames are forever stuck in the TX queues and hang
  the port.

2 (after vsc9959_tas_guard_bands_update): Under the sole goal of
  enabling oversized frame dropping, we make an effort to set
  QSYS_QMAXSDU_CFG_7 to 1230 bytes. But QSYS_QMAXSDU_CFG_7 plays
  one more role, which we did not take into account: per-tc static guard
  band, expressed in L2 byte time (auto-adjusted for FCS and L1 overhead).
  There is a discrepancy between what the driver thinks (that there is
  no guard band, and 100% of min_gate_len[tc] is available for egress
  scheduling) and what the hardware actually does (crops the equivalent
  of QSYS_QMAXSDU_CFG_7 ns out of min_gate_len[tc]). In practice, this
  means that the hardware thinks it has exactly 0 ns for scheduling tc 7.

In both cases, even minimum sized Ethernet frames are stuck on egress
rather than being considered for scheduling on TC 7, even if they would
fit given a proper configuration. Considering the current situation,
with vsc9959_tas_guard_bands_update(), frames between 60 octets and 1230
octets in size are not eligible for oversized dropping (because they are
smaller than QSYS_QMAXSDU_CFG_7), but won't be considered as eligible
for scheduling either, because the min_gate_len[7] (10000 ns) minus the
guard band determined by QSYS_QMAXSDU_CFG_7 (1230 octets * 8 ns per
octet == 9840 ns) minus the guard band auto-added for L1 overhead by
QSYS::HSCH_MISC_CFG.FRM_ADJ (20 octets * 8 ns per octet == 160 octets)
leaves 0 ns for scheduling in the queue system proper.

Investigating the hardware behavior, it becomes apparent that the queue
system needs precisely 33 ns of 'gate open' time in order to consider a
frame as eligible for scheduling to a tc. So the solution to this
problem is to amend vsc9959_tas_guard_bands_update(), by giving the
per-tc guard bands less space by exactly 33 ns, just enough for one
frame to be scheduled in that interval. This allows the queue system to
make forward progress for that port-tc, and prevents it from hanging.

Fixes: 297c4de6 ("net: dsa: felix: re-enable TAS guard band mode")
Reported-by: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>