netdev - Re: [RFC PATCH v3 3/6] timekeeping: Expose API allowing retrival of current clocksource and counter value

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Date:   Wed, 18 Sep 2019 10:29:06 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jianyong Wu <jianyong.wu@....com>, netdev@...r.kernel.org,
        yangbo.lu@....com, john.stultz@...aro.org, tglx@...utronix.de,
        sean.j.christopherson@...el.com, maz@...nel.org,
        richardcochran@...il.com, Mark.Rutland@....com,
        Will.Deacon@....com, suzuki.poulose@....com
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Steve.Capper@....com, Kaly.Xin@....com, justin.he@....com,
        nd@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH v3 3/6] timekeeping: Expose API allowing retrival of
 current clocksource and counter value

On 18/09/19 10:07, Jianyong Wu wrote:
> From Marc Zyngier <maz@...nel.org>
> A number of PTP drivers (such as ptp-kvm) are assuming what the
> current clock source is, which could lead to interesting effects on
> systems where the clocksource can change depending on external events.
> 
> For this purpose, add a new API that retrives both the current
> monotonic clock as well as its counter value.
> 
> From Jianyong Wu: export this API then modules can use it.

See review of patch 4.  ktime_get_snapshot is even better for your
needs, if I'm not mistaken.

Paolo

> Signed-off-by: Marc Zyngier <maz@...nel.org>
> Signed-off-by: Jianyong Wu <jianyong.wu@....com>
> ---
>  include/linux/timekeeping.h |  3 +++
>  kernel/time/timekeeping.c   | 13 +++++++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
> index a8ab0f143ac4..a5389adaa8bc 100644
> --- a/include/linux/timekeeping.h
> +++ b/include/linux/timekeeping.h
> @@ -247,6 +247,9 @@ extern int get_device_system_crosststamp(
>  			struct system_time_snapshot *history,
>  			struct system_device_crosststamp *xtstamp);
>  
> +/* Obtain current monotonic clock and its counter value  */
> +extern void get_current_counterval(struct system_counterval_t *sc);
> +
>  /*
>   * Simultaneously snapshot realtime and monotonic raw clocks
>   */
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 44b726bab4bd..07a0969625b1 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1098,6 +1098,19 @@ static bool cycle_between(u64 before, u64 test, u64 after)
>  	return false;
>  }
>  
> +/**
> + * get_current_counterval - Snapshot the current clocksource and counter value
> + * @sc:	Pointer to a struct containing the current clocksource and its value
> + */
> +void get_current_counterval(struct system_counterval_t *sc)
> +{
> +	struct timekeeper *tk = &tk_core.timekeeper;
> +
> +	sc->cs = READ_ONCE(tk->tkr_mono.clock);
> +	sc->cycles = sc->cs->read(sc->cs);
> +}
> +EXPORT_SYMBOL_GPL(get_current_counterval);
> +
>  /**
>   * get_device_system_crosststamp - Synchronously capture system/device timestamp
>   * @get_time_fn:	Callback to get simultaneous device time and
>