net: dsa: felix: tc-taprio intervals smaller than MTU should send at least one packet
[ Upstream commit 11afdc65 ] 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> Signed-off-by: Sasha Levin <sashal@kernel.org>
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