linux-kernel - Re: [PATCH] clocksource/arm_global_timer: reconfigure clockevents after cpufreq change

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Message-ID: <alpine.DEB.2.20.1611291432290.4358@nanos>
Date:   Tue, 29 Nov 2016 14:42:11 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexander Kochetkov <al.kochet@...il.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>, kernel@...inux.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Patrice Chotard <patrice.chotard@...com>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] clocksource/arm_global_timer: reconfigure clockevents
 after cpufreq change

On Tue, 29 Nov 2016, Alexander Kochetkov wrote:

> After a cpufreq transition, update the clockevent's frequency
> by fetching the new clock rate from the clock framework and
> reprogram the next clock event.

The frequency change would not only affect the clockevent device, it also
would affect the clocksource. So the patch is incomplete, but see below.

> The clock supplying the arm-global-timer on the rk3188 is coming
> from the the cpu clock itself and thus changes its rate everytime
> cpufreq adjusts the cpu frequency.

That's broken and the clk framework should keep the CORE_PERI clock at a
constant rate by reprogramming the divider of the CPU clock.

> Found by code review, real impact not known. Assume what actual
> HZ value will be different from expected on platforms using
> arm-global-timer as clockevent.

Assumptions w/o real impact are a perfect reason not to apply that
patch. This want's a proper proof that the global timer really changes and
this hackery is required, which I seriously doubt.

Thanks,

	tglx