Re: udelay function delays the wrong time interval in multiprocessorsystem, if ARCH_HAS_READ_CURRENT_TIMER is not defined and on current timer is used.

From: chpoph
Date: Mon Mar 18 2013 - 10:41:06 EST

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

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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