netdev - Re: [iwl-net v1 2/2] igb: Fix missing time sync events

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Date: Sat, 17 Feb 2024 07:05:07 -0800
From: Richard Cochran <richardcochran@...il.com>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc: intel-wired-lan@...ts.osuosl.org, sasha.neftin@...el.com,
	kurt@...utronix.de, anthony.l.nguyen@...el.com,
	jesse.brandeburg@...el.com, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [iwl-net v1 2/2] igb: Fix missing time sync events

On Fri, Feb 16, 2024 at 05:04:54PM -0800, Vinicius Costa Gomes wrote:
> Fix "double" clearing of interrupts, which can cause external events
> or timestamps to be missed.
> 
> The E1000_TSIRC Time Sync Interrupt Cause register can be cleared in two
> ways, by either reading it or by writing '1' into the specific cause
> bit. This is documented in section 8.16.1.

That is not what the doc says.

It says this:

    Note: Once ICR.Time_Sync is set, the internal value of this
          register should be cleared by writing 1b to *all* bits
          or cleared by a read to enable receiving an additional
          ICR.Time_Sync interrupt.

    - Intel® Ethernet Controller I210 Datasheet,
      Revision Number: 3.1, June 2017, page 469

It says *all* bits.  This implies that the interrupt sources may be
cleared individually.  Because the non-acked bits are still pending,
the level interrupt should still be active, and the "missing" event
will be handled at the next invocation of the handler.

 
> The following flow was used:
>     1. read E1000_TSIRC into 'tsicr';
>     2. handle the interrupts present into 'tsirc' and mark them in 'ack';
>     3. write 'ack' into E1000_TSICR;
> 
> As both (1) and (3) will clear the interrupt cause, if an interrupt
> happens between (1) and (3) it will be ignored, causing events to be
> missed.

Are you sure?

Does setting ICR.Time_Sync[TXTS] also clear ICR.Time_Sync[RXTS] ?

That is what you seem to be saying.

Thanks,
Richard