linux-kernel - Re: [PATCH v1] PM / Domains: Add tracepoints

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Date:   Tue, 15 Oct 2019 10:19:37 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Douglas Anderson <dianders@...omium.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] PM / Domains: Add tracepoints

Hi Ulf,

On Tue, Oct 15, 2019 at 02:37:42PM +0200, Ulf Hansson wrote:
> On Tue, 1 Oct 2019 at 18:35, Matthias Kaehlcke <mka@...omium.org> wrote:
> >
> > On Fri, Sep 27, 2019 at 04:42:39AM -0400, Steven Rostedt wrote:
> > > On Thu, 26 Sep 2019 15:04:38 -0700
> > > Matthias Kaehlcke <mka@...omium.org> wrote:
> > >
> > > > Define genpd_power_on/off and genpd_set_performance_state
> > > > tracepoints and use them.
> > >
> > > I agree with Greg about adding a "why" you need this. But, in case
> > > there's a good reason to have this, I have comments about the code
> > > below.
> >
> > Thanks Greg and Steven for your comments.
> >
> > How about this instead:
> >
> >   Add tracepoints for genpd_power_on, genpd_power_off and
> >   genpd_set_performance_state. The tracepoints can help with
> >   understanding power domain behavior of a given device, which
> >   may be particularly interesting for battery powered devices
> >   and suspend/resume.
> 
> Apologize for the delay, no excuse!
> 
> I don't mind adding trace events, as long as it's for good reasons -
> and to me, that seems a bit questionable here.
> 
> According to the above, I believe the information you need is already
> available via genpd's debugfs interface, no?

Not in all cases, e.g. you can't peek at sysfs while the device is
suspended. Also sysfs doesn't help much with seeing that a PD is
toggling between on an off for some (possibly legitimate) reason.

At this point I don't need this information badly, just thought it
could be useful. No problem if it is decided to hold back on it for
now.