linux-kernel - Re: [PATCH v2 3/4] clocksource/drivers/exynos

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Date:   Thu, 7 Apr 2022 09:48:29 +0200
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
CC:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        kernel <kernel@...s.com>,
        "alim.akhtar@...sung.com" <alim.akhtar@...sung.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, <krzk@...nel.org>
Subject: Re: [PATCH v2 3/4] clocksource/drivers/exynos_mct: Support
 local-timer-index property

On Wed, Mar 30, 2022 at 10:21:37AM +0200, Vincent Whitchurch wrote:
> On Mon, Mar 21, 2022 at 09:00:08AM +0100, Marek Szyprowski wrote:
> > Right, I've played a bit with MCT on some older Exynos SoCs (ARM 32bit 
> > based and even Exynos5433) and it looked that none of it enabled MCT FRC 
> > timer in their proprietary firmware. I've even proposed a patch for this 
> > once ([1]), but such approach has been rejected. I think that calling 
> > exynos4_mct_frc_start() unconditionally won't hurt.
> 
> Thank you for looking into this.  The proposal was however not to avoid
> changing when exynos4_mct_frc_start() is called, but to instead skip the
> write to the Timer Enable bit of the G_TCON register if it is already
> set, like in the below patch.  (This is needed to avoid races when the
> FRC is shared between CPUs in an AMP configuration, since TCON can be
> modified for other reasons from the CPU which is using the global
> comparator.)
> 
> If I understand your comment correctly, such a change should not cause
> any difference at least on the platforms you looked at since there
> MCT_G_TCON_START will not have been set at startup.

I needed the frc-shared property anyway to prevent registration of the
clock events so I followed Krzysztof's suggestion of doing this
conditionally and also clearing the resume callback.