Guarantee udelay(N) spins at least N microseconds

[slash.tmp@free.fr (Mason)]

On 11/04/2015 09:30, Russell King - ARM Linux wrote:

> On Fri, Apr 10, 2015 at 11:22:56PM +0200, Mason wrote:
>
>> It was you who wrote some time ago: "Timers are preferred
>> because of the problems with the software delay loop."
>> (My system implements DVFS.)
>>
>> It seems to me that a 90 kHz timer is still better than
>> the jiffy counter, or am I mistaken again?
> 
> Given the choice of a 90kHz timer vs using a calibrated software
> delay loop, the software delay loop wins.  I never envisioned that
> someone would be silly enough to think

I'm full of surprises.

> that a 90kHz timer would somehow be suitable to replace a software
> delay loop calibrated against a timer.

Only one message ago, you were arguing that loop-based delays
could be up to 50% inaccurate. Thus, if one wanted to spin for
500 ?s, they'd have to request 1 ms just to be sure. An 11 ?s
accuracy looks like a better deal to me, overall.

Add DVFS to the mix, and that 500 ?s loop-based delay turns into
a 50 ?s delay when the other core decides to boost the cluster
from 100 MHz to 1 GHz. And then drivers break randomly.

Regards.