netdev - Re: igc: missing HW timestamps at TX

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Date:   Tue, 16 Aug 2022 10:51:03 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        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

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.

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.

Thanks,
Kurt

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