[PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems#
nicolas.pitre at linaro.org
Tue May 13 16:29:52 PDT 2014
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
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.
More information about the linux-arm-kernel