linux-kernel - Re: [PATCH] [RFC] clocksource: improve GENERIC

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Message-ID: <20170912091012.GD20805@n2100.armlinux.org.uk>
Date:   Tue, 12 Sep 2017 10:10:13 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        linux-ia64@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Vineet Gupta <vgupta@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC] clocksource: improve GENERIC_CLOCKEVENTS dependency

On Tue, Sep 12, 2017 at 10:09:51AM +0200, Linus Walleij wrote:
> For ARM we now have two subarchs not using generic clockevents:
> RISC PC and EBSA110.
> 
> I think Russell stated these two cannot be converted to generic clockevents
> because of hardware limitations I guess, no timer interrupt, simply, which
> means no clockevents, or unreliable or not granular enough timers.
> 
> IIUC the SA110 does not contain the built-in SoC goodies of the SA1100
> so it needs external timer blocks, and those two machines don't have
> good enough timers.

That's hardly surprising because SA1100 is a SoC, SA110 is just a CPU,
containing no peripherals at all.

EBSA110 only has one usable timer, which must be programmed to produce
a regular timer tick to the OS: it's no good trying to double up the
clocksource and a periodic clockevent onto one counter register - the
clock source will see the same timer value +/- interrupt latency, and
in any case it won't wrap in a power-of-2 manner.

This breaks the assumptions behind the clocksource and timekeeping
code, which are that we have a timer that wraps in a power-of-2
manner, and which takes much longer than the desired period to wrap.

I think RiscPC may be convertable as there are two timers, and I think
the second timer is unused (so could be programmed to the requirements
of a clocksource) but is there much reason to bother given the EBSA110?
I think there isn't.

-- 
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