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: <20130315181449.GX4977@n2100.arm.linux.org.uk>
Date:	Fri, 15 Mar 2013 18:14:49 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	chpoph <chpoph@...il.com>
Cc:	linux-kernel@...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>,
	will.deacon@....com
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 12:08:54AM +0800, chpoph wrote:
> If ARCH_HAS_READ_CURRENT_TIMER is not defined and on current timer is
> used for udelay, then __loop_delay and __loop_const_udelay is used to
> delay a specific time interval.  but in delay-loop.S, loops_per_jiffy
> (not per cpu data) is used to calculate the number of loops. in SMP
> system, udelay delays the wrong time interval if two cpus running at
> different frequency.

We don't support different CPUs running at different frequencies with
the delay loop.  Sorry.
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