Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems# - Russell King

From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Stephen Warren <swarren@wwwdotorg.org>,
	Doug Anderson <dianders@chromium.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Will Deacon <will.deacon@arm.com>,
	John Stultz <john.stultz@linaro.org>,
	David Riley <davidriley@chromium.org>,
	"olof@lixom.net" <olof@lixom.net>,
	Sonny Rao <sonnyrao@chromium.org>,
	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>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems#
Date: Wed, 14 May 2014 00:36:45 +0100	[thread overview]
Message-ID: <20140513233645.GN3693@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <alpine.LFD.2.11.1405131922580.980@knanqh.ubzr>

On Tue, May 13, 2014 at 07:29:52PM -0400, Nicolas Pitre wrote:
> 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.

As I've been saying... use a bloody timer. :)

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