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

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Date:   Tue, 29 Nov 2016 15:32:28 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marc Zyngier <marc.zyngier@....com>
cc:     Alexander Kochetkov <al.kochet@...il.com>,
        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>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] clocksource/arm_global_timer: reconfigure clockevents
 after cpufreq change

On Tue, 29 Nov 2016, Marc Zyngier wrote:
> On 29/11/16 13:42, Thomas Gleixner wrote:
> > 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.
> 
> Well, let's not underestimate the "creativity" [1] of A5/A9 when it
> comes to the timer clocks, and it is a very sad fact that both the
> global timer and the local timers are clocked by PERIPHCLK, which is
> ticking at a fixed ratio N (N >= 2) of the main CPU clock (CLK):
> 
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0407f/CIHGECHJ.html
> 
> I'm not sure how feasible it is to change this ratio (the TRM seems to
> be very silent on the subject).

The CRU documentation of the RK3188 suggests that you can adjust it as it
has a seperate divider, but who knows.

> So short of being able to reconfigure it on the fly, this will probably
> need some surgery similar to what we already do for the TWD (which this
> patch mimics).
>
> Thankfully, we don't see that anymore on moderately recent HW (anything
> since A15) and the advent of the arch timer, which is guaranteed to have
> a fixed frequency.

Can we just disable that global timer on affected SoCs and use something
else instead?

Thanks,

	tglx