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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Date:	Mon, 18 Mar 2013 22:40:56 +0800
From:	chpoph <chpoph@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Will Deacon <will.deacon@....com>,
	"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 Sun, Mar 17, 2013 at 08:05:43PM +0000, Will Deacon wrote:
>Do you actually have an ARM platform that can scale the CPU frequencies
independently?

Yes,  my smart phone use Qualcomm's 8x25 and 8064 platform, which can
scale the CPU frequencies independently. I test the delay loop, the
phone can't get accurate delayed time intervals. I think there are
four reasons: the task scheduling, IRQ handling, memory accessing, and
udelay may be called between CPU frequency changing and
loops_per_jiffy changing. The system's performance decreased greatly
after adding preempt_{enable, disable}.

I used the current timer to delay a time period at last, the timer's
frequency is constant.
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