netdev - Re: igc: missing HW timestamps at TX

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Message-ID: <87v8qrhq7w.fsf@intel.com>
Date:   Tue, 16 Aug 2022 13:45:55 -0700
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Kurt Kanzenbach <kurt@...utronix.de>,
        Vladimir Oltean <vladimir.oltean@....com>
Cc:     Ferenc Fejes <ferenc.fejes@...csson.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "marton12050@...il.com" <marton12050@...il.com>,
        "peti.antal99@...il.com" <peti.antal99@...il.com>
Subject: Re: igc: missing HW timestamps at TX

Kurt Kanzenbach <kurt@...utronix.de> writes:

> Hi Vinicius,
>
> On Mon Aug 15 2022, Vinicius Costa Gomes wrote:
>> I think your question is more "why there's that workqueue on igc?"/"why
>> don't you retrieve the TX timestamp 'inline' with the interrupt?", if I
>> got that right, then, I don't have a good reason, apart from the feeling
>> that reading all those (5-6?) registers may take too long for a
>> interrupt handler. And it's something that's being done the same for
>> most (all?) Intel drivers.
>
> We do have one optimization for igb which attempts to read the Tx
> timestamp directly from the ISR. If that's not ready *only* then we
> schedule the worker. I do assume igb and igc have the same logic for
> retrieving the timestamps here.
>

That seems a sensible approach. And yes, the timestamping logic is the
same.

> The problem with workqueues is that under heavy system load, it might be
> deferred and timestamps will be lost. I guess that workqueue was added
> because of something like this: 1f6e8178d685 ("igb: Prevent dropped Tx
> timestamps via work items and interrupts.").
>
>>
>> I have a TODO to experiment with removing the workqueue, and retrieving
>> the TX timestamp in the same context as the interrupt handler, but other
>> things always come up.
>
> Let me know if you have interest in that igb patch.

That would be great! Thanks.

>
> Thanks,
> Kurt


Cheers,
-- 
Vinicius