linux-kernel - Re: Time: new clocksource

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Message-ID: <CAHhAz+iZe4A9bi9tvwqpHaV1ari76r2bXcO1E6Bm88Cc7XFq+A@mail.gmail.com>
Date:   Tue, 7 Dec 2021 13:03:07 +0530
From:   Muni Sekhar <munisekharrms@...il.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     tglx@...utronix.de, LKML <linux-kernel@...r.kernel.org>,
        kernelnewbies <kernelnewbies@...nelnewbies.org>
Subject: Re: Time: new clocksource

On Sat, Dec 4, 2021 at 3:18 AM Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>
> Hi Sekhar,
>
> On 03/12/2021 17:50, Muni Sekhar wrote:
> > Hi All,
> >
> > We have a Digital PLL with 64 bit timer counter hardware and the
> > counter is accessible from the CPU over the PCIe bus.
> >
> > Is it possible to add this timer counter hardware as new clocksource
> > driver? To do this, can someone please point me to the existing
> > reference source code(or patch) for this task.
>
> You can refer to the drivers located in drivers/clocksource
>
> git annotate on one of the driver can give you the initial commit where
> recent submissions explain the timer internals.
>
> > Suppose if it is possible to add a new clocksource driver for this
> > hardware then does any userspace get_timestamp* API would get the time
> > from this new hardware?
>
> It should if the timer is selected as the clocksource
/var/log/kern.log file use a absolute time, like "Nov 29 12:26:13", as
well as relative time in seconds and microseconds since startup , like
"[1263900.984287]

Is the relative time calculated by the clocksource driver module (tsc
or acpi_pm)?

Which module is responsible for resulting in the absolute time? Is
absolute time synchronized across multiple systems connected on the
network?
>
>
> --
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-- 
Thanks,
Sekhar