Subject: Re: [Intel-wired-lan] [PATCH iwl-net v3] igc: Restore default Qbv schedule when changing channels

From: Vinicius Costa Gomes <[email-masked]>
Date: Mon, 05 Jan 2026 14:39:43 -0800

Kurt Kanzenbach <[email-masked]> 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 <[email-masked]>
> ---
> Changes in v3:
> - Adjust commit message and comments (Aleksandr)
> - Link to v2: https://lore.kernel.org/r/[email-masked]
>
> 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/[email-masked]
> ---

Acked-by: Vinicius Costa Gomes <[email-masked]>

Cheers,
-- 
Vinicius