linux-kernel - Re: [PATCH v3 0/8] NVIDIA Tegra clocksource driver improvements

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Date:   Mon, 3 Jun 2019 10:17:50 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>
CC:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joseph Lo <josephl@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Nicolas Chauvet <kwizart@...il.com>
Subject: Re: [PATCH v3 0/8] NVIDIA Tegra clocksource driver improvements

On Fri, May 31, 2019 at 03:33:41PM +0300, Dmitry Osipenko wrote:
> 31.05.2019 11:26, Peter De Schrijver пишет:
> > On Fri, May 24, 2019 at 06:32:45PM +0300, Dmitry Osipenko wrote:
> >> Hello,
> >>
> >> This series primarily unifies the driver code across all Tegra SoC
> >> generations. In a result the clocksources are allocated per-CPU on
> >> older Tegra's and have a higher rating than the arch-timer, the newer
> >> Tegra210 is getting support for microsecond clocksource and the driver's
> >> code is getting much cleaner. Note that arch-timer usage is discouraged on
> >> all Tegra's due to the time jitter caused by the CPU frequency scaling.
> > 
> > I think the limitations are more as follows:
> > 
> > Chip	timer		suffers cpu dvfs jitter		can wakeup from cc7
> > T20	us-timer	No				Yes
> > T20	twd timer	Yes				No?
> > T30	us-timer	No				Yes
> > T30	twd timer	Yes				No?
> > T114	us-timer	No				Yes
> > T114	arch timer	No				Yes
> > T124	us-timer	No				Yes
> > T124	arch timer	No				Yes
> > T210	us-timer	No				Yes
> > T210	arch timer	No				No
> > T210	clk_m timer	No				Yes
> > 
> > right?
> 
> Doesn't arch timer run off the CPU clock? If yes (that's what I
> assumed), then it should be affected by the DVFS. Otherwise I'll lower
> the clocksource's rating for T114/124/132.
> 

No. It doesn't. This is the big change between A9 and later CPUs. 

Peter.

> TWD can't wake CPU from the power-down state, so it's a solid "No" for
> TWD in the "can wakeup from cc7" column.