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

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Message-ID: <20140508205556.GJ3693@n2100.arm.linux.org.uk>
Date:	Thu, 8 May 2014 21:55:56 +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>
Subject: Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP
	systems

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.

When you start from that point, rather than (as you seem to be) believing
that it has some kind of accuracy, then the implementation of scaling the
loops_per_jiffy is entirely reasonable - it's a best effort implementation.

Again, just use a timer for your udelay() implementation.

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