linux-kernel - 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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Date:	Sat, 16 Mar 2013 00:08:54 +0800
From:	chpoph <chpoph@...il.com>
To:	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>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	will.deacon@....com
Subject: 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.

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.


In arch/arm/lib/delay.c arm_delay_ops is set to arm_delay_ops.
/*
 * Default to the loop-based delay implementation.
 */
struct arm_delay_ops arm_delay_ops = {
        .delay          = __loop_delay,
        .const_udelay   = __loop_const_udelay,
        .udelay         = __loop_udelay,
};
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