linux-kernel - Re: udelay function delays the wrong time interval in multiprocessor system, if ARCH_HAS_READ_CURRENT_TIMER is not defined and on current timer is used.

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Message-ID: <20130317200543.GA20174@mudshark.cambridge.arm.com>
Date:	Sun, 17 Mar 2013 20:05:43 +0000
From:	Will Deacon <will.deacon@....com>
To:	chpoph <chpoph@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Nicolas Pitre <nico@...aro.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: udelay function delays the wrong time interval in multiprocessor
 system, if ARCH_HAS_READ_CURRENT_TIMER is not defined and on current timer
 is used.

On Sat, Mar 16, 2013 at 03:32:43AM +0000, chpoph wrote:
> On Sat, Mar 16, 2013 at 2:14 AM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > We don't support different CPUs running at different frequencies with
> > the delay loop.  Sorry.
> 
> Does it means that a timer-based delay implementation must be used to
> get an accurate delay in SMP. I think it should print a warning
> message if the CPU delay loop is used in SMP.  In my system, the wrong
> delay interval fluctuated with CPU frequencies caused a control
> problem.

I've been playing around with loops_per_jiffy recently, in an attempt to
clean up the cpufreq scaling code so that the SMP-ness is in core code,
rather than being duplicated by every architecture:

  git://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git lpj

With those changes, it's pretty easy to get different delays depending on
the current CPU, but it would require preempt_{enable,disable} calls around
the delay, which I haven't convinced myself about.

Do you actually have an ARM platform that can scale the CPU frequencies
independently?

Will
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