linux-kernel - Re: [RFC/PATCH 00/11] arm: omap: counter32k rework

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Date:	Mon, 05 Oct 2015 14:19:35 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Tony Lindgren <tony@...mide.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	balbi@...com, Thierry Reding <thierry.reding@...il.com>,
	linux-tegra@...r.kernel.org, tglx@...utronix.de,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>
Subject: Re: [RFC/PATCH 00/11] arm: omap: counter32k rework

On Monday 05 October 2015 04:13:41 Tony Lindgren wrote:
> * Arnd Bergmann <arnd@...db.de> [151005 04:08]:
> > On Monday 05 October 2015 03:55:55 Tony Lindgren wrote:
> > > * Daniel Lezcano <daniel.lezcano@...aro.org> [151001 15:16]:
> > > > On 09/30/2015 04:49 PM, Arnd Bergmann wrote:
> > > > >On Wednesday 30 September 2015 16:42:21 Arnd Bergmann wrote:
> > > > >>
> > > > >>TEGRA folks: the tegra_read_persistent_clock() implementation apparently
> > > > >>predates the Tegra RTC driver and I wonder if they actually do the
> > > > >>right thing in combination. Could it be that the wall time forwards
> > > > >>twice as fast as it should during resume when the RTC driver is loaded?
> > > > >>Could it be that we can simply remove  tegra_read_persistent_clock()
> > > > >>and the register_persistent_clock() infrastructure?
> > > > >>
> > > > >
> > > > >I found the 'sleeptime_injected' variable now, which takes care of
> > > > >forwarding the clock by the correct amount.
> > > > >
> > > > >I also found the CLOCK_SOURCE_SUSPEND_NONSTOP flag next to it, which
> > > > >should let us use the counter32k driver to provide the correct
> > > > >time during suspend without the omap_read_persistent_clock() function.
> > > > >We should be able to just delete that code.
> > > > >
> > > > >If we decide to also delete the tegra_read_persistent_clock()
> > > > >function, we can remove the registration too.
> > > > 
> > > > 
> > > > +1
> > > 
> > > We could maybe have read_persistent_clock() just check for the
> > > CLOCK_SOURCE_SUSPEND_NONSTOP flag?
> > 
> > timekeeping_resume() already ignores the persistent clock values if
> > the clocksource has this set. Do you mean we should additionally
> > not call the read_persistent_clock() function at all to safe a
> > few cycles reading that value?
> 
> Hmm no I mean if we have CLOCK_SOURCE_SUSPEND_NONSTOP we can
> automatically make read_persistent_clock() use that if nothing
> else got registered.

Ok, so we don't need to change anything here then.

> > How expensive is the function?
> 
> Usually the persistent clock is on some interconnect, so it is
> way slower compared to a local timer. It doubt it makes a
> difference in timekeeping_resume() though 

Right.

	Arnd
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