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

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Date:	Thu, 8 May 2014 17:02:02 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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

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.
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