linux-kernel - Re: [PATCH/RFC] msleep() with hrtimers

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Date:	Mon, 16 Jul 2007 08:57:22 -0700
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"Roman Zippel" <zippel@...ux-m68k.org>
Cc:	"Jonathan Corbet" <corbet@....net>, linux-kernel@...r.kernel.org,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...e.hu>
Subject: Re: [PATCH/RFC] msleep() with hrtimers

On 7/16/07, Roman Zippel <zippel@...ux-m68k.org> wrote:
> On Mon, 16 Jul 2007, Jonathan Corbet wrote:
> > > One possible problem here is that setting up that timer can be
> > > considerably more expensive, for a relative timer you have to read the
> > > current time, which can be quite expensive (e.g. your machine now uses the
> > > PIT timer, because TSC was deemed unstable).
> >
> > That's a possibility, I admit I haven't benchmarked it.  I will say that
> > I don't think it will be enough to matter - msleep() is not a hot-path
> > sort of function.  Once the system is up and running it almost never
> > gets called at all - at least, on my setup.
>
> That's a bit my problem - we have to consider other setups as well.
> Is it worth converting all msleep users behind their back or should we
> just a provide a separate function for those who care?

As a driver author (2.4 timeframe, embedded platform, see gitinc.com
for the hardware description), I would rather msleep did what it says
it's going to do. If the current one can wait 20 times longer than you
ask for, then that's just broken.

> > > One question here would be, is it really a problem to sleep a little more?
> >
> > "A little more" is a bit different than "twenty times as long as you
> > asked for."  That "little bit more" added up to a few seconds when
> > programming a device which needs a brief delay after tweaking each of
> > almost 200 registers.
>
> Which driver is this? I'd like to look at this, in case there's some other
> hidden problem.

"Hidden problem"? Lots of hardware has quiescent wait-time
requirements, to guarantee that whatever is going on it it's little
state machine head finally reaches a stable state. Waiting for those
changes to take effect before starting the next set of register
reprogramming is a common requirement -- on the hardware I've worked
with at least.

Ray
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