Subject: [PATCH v2 2/2] ARM: delay: allow timer-based delay implementation to be selected

From: [email-masked] (Shinya Kuribayashi)
Date: Thu, 12 Jul 2012 18:35:37 +0900

On 7/12/2012 5:44 PM, Will Deacon wrote:
>> On 6/30/2012 2:33 AM, Will Deacon wrote:
>>> +void __init init_current_timer_delay(unsigned long freq)
>>> +{
>>> +	pr_info("Switching to timer-based delay loop\n");
>>> +	lpj_fine			= freq / HZ;
>>> +	arm_delay_ops.delay		= __timer_delay;
>>> +	arm_delay_ops.const_udelay	= __timer_const_udelay;
>>> +	arm_delay_ops.udelay		= __timer_udelay;
>>> +}
>>
>> Once this function is processed, the udelay() behavior changes
>> _immediately_ from loop-based delay to timer-based one, without waiting
>> for 'loops_per_jiffy' itself being corrected in calibrate_delay().
>>
>> As a result, actual udelay()s may be toooo long than expected, in
>> particular udelay()s used between init_current_timer_delay() and
>> calibrate_delay().  It's unlikely be short, as the frequency of a
>> counter for read_current_timer is typically slower than CPU frequency.
> 
> Surely using udelay before calibrate_delay_loop has been called is a
> fundamental error?

Got it.  I'm just not confident about disallowing early use of udelay().

  Shinya