linux-kernel - Re: [PATCH] powerpc/time: set ->min_delta_ticks and ->max_delta

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Message-ID: <874lya5erp.fsf@concordia.ellerman.id.au>
Date:   Fri, 31 Mar 2017 14:03:22 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicolai Stange <nicstange@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
        Nicolai Stange <nicstange@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oliver O'Halloran <oohall@...il.com>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/time: set ->min_delta_ticks and ->max_delta_ticks

Nicolai Stange <nicstange@...il.com> writes:

> In preparation for making the clockevents core NTP correction aware,
> all clockevent device drivers must set ->min_delta_ticks and
> ->max_delta_ticks rather than ->min_delta_ns and ->max_delta_ns: a
> clockevent device's rate is going to change dynamically and thus, the
> ratio of ns to ticks ceases to stay invariant.
>
> Make the powerpc arch's clockevent driver initialize these fields properly.
>
> This patch alone doesn't introduce any change in functionality as the
> clockevents core still looks exclusively at the (untouched) ->min_delta_ns
> and ->max_delta_ns. As soon as this has changed, a followup patch will
> purge the initialization of ->min_delta_ns and ->max_delta_ns from this
> driver.
>
> Signed-off-by: Nicolai Stange <nicstange@...il.com>
> ---
>
> Notes:
>     This prerequisite patch is part of a larger effort to feed NTP
>     corrections into the clockevent devices' frequencies and thus
>     avoiding their notion of time to diverge from the system's
>     one. If you're interested, the current state of the whole series
>     can be found at [1].
>     
>     If you haven't got any objections and these prerequisites get
>     merged by 4.12 everywhere, I'll proceed with the remainder of
>     this series in 4.13.
>     
>     Applicable to next-20170324 as well as to John' Stultz tree [2].
>     
>     [1]
>       git://nicst.de/linux.git cev-freq-adj.v10.fortglx-4.12-time
>       https://nicst.de/git/?p=linux.git;a=shortlog;h=refs/heads/cev-freq-adj.v10.fortglx-4.12-time
>     
>     [2]
>       https://git.linaro.org/people/john.stultz/linux.git fortglx/4.12/time
>
>  arch/powerpc/kernel/time.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 07b90725855e..2b33cfaac7b8 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -995,8 +995,10 @@ static void __init init_decrementer_clockevent(void)
>  
>  	decrementer_clockevent.max_delta_ns =
>  		clockevent_delta2ns(decrementer_max, &decrementer_clockevent);
> +	decrementer_clockevent.max_delta_ticks = decrementer_max;
>  	decrementer_clockevent.min_delta_ns =
>  		clockevent_delta2ns(2, &decrementer_clockevent);
> +	decrementer_clockevent.min_delta_ticks = 2;

Seems to do what it says on the tin.

Acked-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)


cheers