linux-kernel - Re: [PATCH 04/18] power: ab8500_fg: Replace msleep() with usleep

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Message-Id: <201301151323.28879.arnd@arndb.de>
Date:	Tue, 15 Jan 2013 13:23:28 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	Joe Perches <joe@...ches.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linus.walleij@...ricsson.com, cbouatmailru@...il.com,
	Jonas Aaberg <jonas.aberg@...ricsson.com>
Subject: Re: [PATCH 04/18] power: ab8500_fg: Replace msleep() with usleep_range() for greater accuracy

On Tuesday 15 January 2013, Lee Jones wrote:
> > > @@ -956,7 +956,7 @@ static int ab8500_fg_load_comp_volt_to_capacity(struct ab8500_fg *di)
> > >     do {
> > >             vbat += ab8500_fg_bat_voltage(di);
> > >             i++;
> > > -           msleep(5);
> > > +           usleep_range(5000, 5001);
> > 
> > If you're going to give a range that small
> > you might as well use usleep instead.
> > 
> > Otherwise, add some tolerance to allow any
> > other coalesced wakeup to occur.
> 
> I can't increase the tolerance, as I don't know how that would
> effect the running of the system, and the person who would know
> is off on parental leave.

The function only averages the voltage between a couple of readings.
It won't change much if those register reads are slightly more
uniformly timed. Note that the thread can still be preempted for
a much longer time if anything else is running, and the entire
interrupt handling in this driver looks so fragile that I would
not rely on the interrupt actually happening at the right time
anyway. I think it should first be debugged properly to remove
the need for the enable_irq/disable_irq calls.

	Arnd
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