linux-kernel - Re: Kernel without RTC

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Message-ID: <alpine.DEB.2.20.1703081602460.3521@nanos>
Date:   Wed, 8 Mar 2017 16:06:23 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>
cc:     "Hadimani, Jagadish" <Jagadish.Hadimani@....com>,
        "Shah, Nehal-bakulchandra" <Nehal-bakulchandra.Shah@....com>,
        "a.zummo@...ertech.it" <a.zummo@...ertech.it>,
        "rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: Kernel without RTC

On Wed, 8 Mar 2017, Alexandre Belloni wrote:
> On 08/03/2017 at 13:33:33 +0000, Hadimani, Jagadish wrote:
> > Hello Alexandre,
> > 
> > I guess the Linux kernel uses HPET timer...
> > But can we can force Linux kernel to use Tsc or per core timer...
> > 
> 
> That is probably the case but your are targeting the wrong subsystem.
> The timekeeping is done using two different devices: clocksource and
> clockevent. Usually, the drivers are in drivers/clocksource. I'm
> definitively not an expert in x86 but the clockevent seems to be
> registered from arch/x86/kernel/hpet.c and the clocksource from
> arch/x86/kernel/tsc.c
> 
> IIRC the clocksource is optional but the clockevent is mandatory so if
> you don't have an HPET, you will need to register a clockevent device
> from somewhere else.
> 
> Maybe the simplest thing to do is to ask the x86 maintainers and the
> time maintainers. Luckily for you, they are the same people (Thomas and
> Ingo, added in cc).

The scheduler uses TSC anyway.

The RTC is only used for setting the wall clock time at boot and for
figuring out the time a system spent in suspend. If you don't have an RTC
then the boot wall clock time will be simply 1/1/1970 00:00:00.

Jagadish, can you please explain what your problem is. Does the system fail
to boot or does it behave strange or what?

Thanks,

	tglx