linux-kernel - Re: [PATCH] clocksource: Add debugfs support

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Date:   Tue, 31 Mar 2020 15:44:25 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clocksource: Add debugfs support

On Tue, Mar 31, 2020 at 3:25 PM Thierry Reding <thierry.reding@...il.com> wrote:
> On Tue, Mar 31, 2020 at 02:50:47PM -0700, John Stultz wrote:
> > On Tue, Mar 31, 2020 at 2:40 PM Thierry Reding <thierry.reding@...il.com> wrote:
> > >
> > > From: Thierry Reding <treding@...dia.com>
> > >
> > > Add a top-level "clocksource" directory to debugfs. For each clocksource
> > > registered with the system, a subdirectory will be added with attributes
> > > that can be queried to obtain information about the clocksource.
> > >
> >
> > Curious, what's the need/planned use for this?  I know in the past
> > folks have tried to get timekeeping internals exported to userland so
> > they could create their own parallel userland timekeeping system,
> > which I worry is a poor idea.
>
> This was meant to be purely for debugging purposes. That is as an easy
> way to check that the code was working and that the counter is properly
> updated.
>
> I certainly wasn't planning on implementing any userland on top of this.
> Well, I guess it could be useful to use these values in test scripts
> perhaps, since one of the clock sources exposed by one of the drivers I
> have been working on is used across Tegra SoCs for hardware
> timestamping. For that it might be interesting to be able to compare
> those timestamp snapshots to something that I can read from userspace
> during testing.

So, other platforms do similar, but utilizing the ktime_get_snapshot()
interface internally so drivers can share that SoC hardware
timestamping logic and export that via driver interfaces in a cleanly
abstracted way to userland, rather than exposing the timekeping
internals.

thanks
-john