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Date: Tue, 13 May 2014 19:29:52 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@linaro.org>
To: Stephen Warren <swarren@wwwdotorg.org>
cc: Doug Anderson <dianders@chromium.org>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Viresh Kumar <viresh.kumar@linaro.org>,
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        Santosh Shilimkar <santosh.shilimkar@ti.com>,
        Shawn Guo <shawn.guo@linaro.org>, Stephen Boyd <sboyd@codeaurora.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Stephen Warren <swarren@nvidia.com>,
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Subject: Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP
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On Tue, 13 May 2014, Nicolas Pitre wrote:

> On Tue, 13 May 2014, Stephen Warren wrote:
> 
> > On 05/13/2014 03:50 PM, Doug Anderson wrote:
> > ...
> > > ...but then I found the true problem shows up when we transition
> > > between very low frequencies on exynos, like between 200MHz and
> > > 300MHz.  While transitioning between frequencies the system
> > > temporarily bumps over to the "switcher" PLL running at 800MHz while
> > > waiting for the main PLL to stabilize.  No CPUFREQ notification is
> > > sent for that.  That means there's a period of time when we're running
> > > at 800MHz but loops_per_jiffy is calibrated at between 200MHz and
> > > 300MHz.
> > > 
> > > 
> > > I'm welcome to any suggestions for how to address this.  It sorta
> > > feels like it would be a common thing to have a temporary PLL during
> > > the transition, ...
> > 
> > We definitely do that on Tegra for some cpufreq transitions.
> 
> Ouch...  If this is a common strategy to use a third frequency during a 
> transition phase, especially if that frequency is way off (800MHz vs 
> 200-300MHz) then it is something the cpufreq layer must capture and 
> advertise.

Of course if only the loops_per_jiffy scaling does care about frequency 
changes these days, and if in those cases udelay() can instead be moved 
to a timer source on those hick-up prone platforms, then all this is 
fairly theoretical and may not be worth pursuing.


Nicolas