Kurt Kanzenbach [off-list ref] writes:

The Multi-queue Priority (MQPRIO) and Earliest TxTime First (ETF) offloads
utilize the Time Sensitive Networking (TSN) Tx mode. This mode is always
coupled to IEEE 802.1Qbv time aware shaper (Qbv). Therefore, the driver
sets a default Qbv schedule of all gates opened and a cycle time of
1s. This schedule is set during probe.

However, the following sequence of events lead to Tx issues:

 - Boot a dual core system
   igc_probe():
     igc_tsn_clear_schedule():
       -> Default Schedule is set
       Note: At this point the driver has allocated two Tx/Rx queues, because
       there are only two CPUs.

 - ethtool -L enp3s0 combined 4
   igc_ethtool_set_channels():
     igc_reinit_queues()
       -> Default schedule is gone, per Tx ring start and end time are zero

  - tc qdisc replace dev enp3s0 handle 100 parent root mqprio \
      num_tc 4 map 3 3 2 2 0 1 1 1 3 3 3 3 3 3 3 3 \
      queues 1@0 1@1 1@2 1@3 hw 1
    igc_tsn_offload_apply():
      igc_tsn_enable_offload():
        -> Writes zeros to IGC_STQT(i) and IGC_ENDQT(i), causing Tx to stall/fail

Therefore, restore the default Qbv schedule after changing the number of
channels.

Furthermore, add a restriction to not allow queue reconfiguration when
TSN/Qbv is enabled, because it may lead to inconsistent states.

Fixes: c814a2d2d48f ("igc: Use default cycle 'start' and 'end' values for queues")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
---
Changes in v3:
- Adjust commit message and comments (Aleksandr)
- Link to v2: https://lore.kernel.org/r/20251118-igc_mqprio_channels-v2-1-c32563dede2f@linutronix.de (local)

Changes in v2:
- Explain abbreviations (Aleksandr)
- Only clear schedule if no error happened (Aleksandr)
- Add restriction to avoid inconsistent states (Vinicius)
- Target net tree (Vinicius)
- Link to v1: https://lore.kernel.org/r/20251107-igc_mqprio_channels-v1-1-42415562d0f8@linutronix.de (local)
---

Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>



Cheers,
-- 
Vinicius