linux-kernel - Re: [PATCHv2 1/3] ARM: Translate delay.S into (mostly) C

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Date:	Wed, 03 Nov 2010 22:40:14 -0400 (EDT)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Daniel Walker <dwalker@...eaurora.org>
Cc:	Stephen Boyd <sboyd@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Saravana Kannan <skannan@...eaurora.org>,
	Colin Cross <ccross@...roid.com>,
	Kevin Hilman <khilman@...prootsystems.com>,
	Santosh Shilimkar <santosh.shilimkar@...com>
Subject: Re: [PATCHv2 1/3] ARM: Translate delay.S into (mostly) C

On Wed, 3 Nov 2010, Daniel Walker wrote:

> On Thu, 2010-10-28 at 14:19 -0700, Stephen Boyd wrote:
> > We want to allow machines to override the __delay() implementation
> > at runtime so they can use a timer based __delay() routine. It's
> > easier to do this using C, so let's write udelay and friends in C.
> > 
> > We lose the #if 0 code, which according to Russell is used "to
> > make the delay loop more stable and predictable on older CPUs"
> > (see http://article.gmane.org/gmane.linux.kernel/888867 for more
> > info). We shouldn't be too worried though, since we'll soon add
> > functionality allowing a machine to set the __delay() loop
> > themselves, thus allowing machines to resurrect the commented out
> > code should they need it.
> > 
> > Nico expressed concern that fixed lpj cmdlines will break due to
> > compiler optimizations. That doesn't seem to be the case since
> > before and after this patch I get the same lpj value when running
> > my CPU at 19.2 MHz. That should be sufficiently slow enough to
> > cover any machine running Linux.
> 
> Nico, are you ready to sign off on this?

Acked-by: Nicolas Pitre <nicolas.pitre@...aro.org>

The compiled code looks trivial enough.  I don't think gcc will find 
ways to optimize it further.  And if gcc regresses then the delay would 
just be longer.


Nicolas
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