linux-kernel - Re: Extreme time jitter with suspend/resume cycles

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Date:   Thu, 5 Oct 2017 09:46:06 -0700
From:   Gabriel Beddingfield <gabe@...tlabs.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        linux-rtc@...r.kernel.org, Guy Erb <guy@...tlabs.com>,
        Howard Harte <hharte@...tlabs.com>
Subject: Re: Extreme time jitter with suspend/resume cycles

Hi John,

First, my apologies for calling it a "hack." I just went back and looked at the
commit history and this is first-class stuff... and you explained it very well
(including the NTP interaction) in the commit message. I'm pretty sure I
read this before, but I reckon most of it went over my head and I garbled it.

On Wed, Oct 4, 2017 at 5:20 PM, John Stultz <john.stultz@...aro.org> wrote:
> Yea. I thought arm devices often had read_persistent_clock64() backed
> by the 32k timer (which is poor for time initialization but works well
> for suspend timing).
>
> Maybe I misunderstood on the first read. Is it then that the
> relatively fine-grained read_persistent_clock64() is colliding with
> the delta_delta logic that assumes we get coarse 1sec resolution? In
> that case the huristic above seems sane.

Yes, exactly.

-gabe