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

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Message-ID: <c686aae8-3be0-805e-265b-a7f16f2a6c02@gmail.com>
Date:   Fri, 31 May 2019 15:33:41 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Peter De Schrijver <pdeschrijver@...dia.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

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.

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.