linux-kernel - Re: [PATCH] ARM: Don't ever downscale loops_per

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Message-ID: <20140509002334.GK3693@n2100.arm.linux.org.uk>
Date:	Fri, 9 May 2014 01:23:35 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Will Deacon <will.deacon@....com>,
	John Stultz <john.stultz@...aro.org>,
	David Riley <davidriley@...omium.org>,
	"olof@...om.net" <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Richard Zhao <richard.zhao@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Shawn Guo <shawn.guo@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Stephen Warren <swarren@...dia.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mark Brown <broonie@...aro.org>
Subject: Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP
	systems

On Thu, May 08, 2014 at 05:02:02PM -0700, Doug Anderson wrote:
> Russel,
> 
> On Thu, May 8, 2014 at 1:55 PM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > On Thu, May 08, 2014 at 11:06:24AM -0700, Doug Anderson wrote:
> >> I guess I would say that my patch is unhacking the this code.  The
> >> code after my patch is simpler.  I would perhaps argue that (ec971ea
> >> ARM: add cpufreq transiton notifier to adjust loops_per_jiffy for smp)
> >> should never have landed to begin with.
> >
> > That depends on your point of view.  As I've already pointed out through
> > the examples of why udelay() is inaccurate, for driver authors, they
> > should assume that udelay() just gives you an "approximate" delay and
> > it has no accuracy.
> 
> That disagrees with what Thomas Gleixner says at
> <http://lkml.iu.edu//hypermail/linux/kernel/1203.1/01034.html>.  It
> also seems like perhaps the regulator core is broken, then...  If a
> udelay(30) can end up as a udelay(20) then we may return from a
> regulator code 10us earlier than we should and we'll assume that a
> regulator is ramped before it really is...
> 
> I'm out tomorrow but I can confirm on Monday that I was really seeing
> udelay(30) be a udelay(20) without this patch.

Thomas is wrong - when I researched this topic, I ended up finding
that udelay() does delay _less_ than requested, and I mailed Linus
about it...  This is whe way udelay() is - it's an approximate delay,
it's *not* accurate.

It's also fairly obvious when you stop and consider how it's calibrated.

Take a moment to wonder when we used to recalibrate each CPU individally,
why the boot CPU bogomips would be slightly lower than the secondary
CPU bogomips.  This is all down to the boot CPU having to run timer
interrupts while the secondary CPUs weren't at the time they calibrated.

Linus doesn't give a damn about udelay() being slightly short in this
way.  Neither do I.

Let me repeat again: use a timer.

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