linux-kernel - RE: [PATCH 3/5] Add calls to translate Always Running Timer (ART) to system time

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Date:	Wed, 29 Jul 2015 02:05:12 +0000
From:	"Hall, Christopher S" <christopher.s.hall@...el.com>
To:	'John Stultz' <john.stultz@...aro.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH 3/5] Add calls to translate Always Running Timer (ART)
 to system time



> -----Original Message-----
> From: John Stultz [mailto:john.stultz@...aro.org]
> Sent: Monday, July 27, 2015 9:11 PM
> To: Hall, Christopher S
> Cc: Thomas Gleixner; Richard Cochran; Ingo Molnar; Kirsher, Jeffrey T;
> Ronciak, John; H. Peter Anvin; x86@...nel.org; lkml;
> netdev@...r.kernel.org
> Subject: Re: [PATCH 3/5] Add calls to translate Always Running Timer
> (ART) to system time
> 
> On Mon, Jul 27, 2015 at 5:46 PM, Christopher Hall
> <christopher.s.hall@...el.com> wrote:
> > +static bool checked_art_to_tsc(cycle_t *tsc)
> > +{
> > +       if (!has_art())
> > +               return false;
> > +       *tsc = art_to_tsc(*tsc);
> > +       return true;
> > +}
> > +
> > +static int art_to_rawmono64(struct timespec64 *rawmono, cycle_t art)
> > +{
> > +       if (!checked_art_to_tsc(&art))
> > +               return -ENXIO;
> > +       return tsc_to_rawmono64(rawmono, art);
> > +}
> > +EXPORT_SYMBOL(art_to_rawmono64);
> 
> This all seems to assume the TSC is the current clocksource, which it
> may not be if the user has overridden it.

I don't make that assumption.  The counter_to_* functions take a
pointer to a clocksource struct.  They return -ENXIO if that clocksource
doesn’t match the current clocksource.

The tsc_to_* functions pass the tsc clocksource pointer to the counter_to_*
functions.  These tsc conversion functions are called by the art_to_*
functions.

> 
> If instead there were a counter_to_rawmono64() which took the counter
> value and maybe the name of the clocksource (if the strncmp is
> affordable for your use), it might be easier for the core to provide
> an error if the current timekeeping clocksource isn't the one the
> counter value is based on. This would also allow the tsc_to_*()
> midlayers to be dropped (since they don't seem to do much).
> 
> thanks
> -john

Again, thanks for your input.

Chris