netdev - Re: [PATCH iwl-next] igc: Restore default Qbv schedule when changing channels

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Message-ID: <87bjl6l3j5.fsf@jax.kurt.home>
Date: Thu, 13 Nov 2025 09:50:38 +0100
From: Kurt Kanzenbach <kurt@...utronix.de>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>, Tony Nguyen
 <anthony.l.nguyen@...el.com>, Przemek Kitszel
 <przemyslaw.kitszel@...el.com>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
 <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Sebastian Andrzej
 Siewior <bigeasy@...utronix.de>, intel-wired-lan@...ts.osuosl.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH iwl-next] igc: Restore default Qbv schedule when
 changing channels

On Wed Nov 12 2025, Vinicius Costa Gomes wrote:
> Hi,
>
> Kurt Kanzenbach <kurt@...utronix.de> writes:
>
>> The MQPRIO (and ETF) offload utilizes the TSN Tx mode. This mode is always
>> coupled to 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
>>    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 CPU(s).
>>
>>  - 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) -> Boom
>>
>> Therefore, restore the default Qbv schedule after changing the amount of
>> channels.
>>
>
> Couple of questions:
>  - Would it make sense to mark this patch as a fix?

This only happens if a user uses ETF or MQPRIO and a dual/single core
system. So I didn't see the need to mark it as a fix.

>
>  - What would happen if the user added a Qbv schedule (not the default
>    one) and then changed the number of queues? My concern is that 'tc
>    qdisc' would show the custom user schedule and the hardware would be
>    "running" the default schedule, this inconsistency is not ideal. In
>    any case, it would be a separate patch.

Excellent point. Honestly I'm not sure what to expect when changing the
number of queues after a user Qbv schedule is added. For MQPRIO we added
a restriction [1] especially for that case. I'm leaning towards the same
solution here. What do you think?

Thanks,
Kurt

[1] - https://elixir.bootlin.com/linux/v6.18-rc5/source/drivers/net/ethernet/intel/igc/igc_ethtool.c#L1564

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