Re: [RFC] Improving udelay/ndelay on platforms where that is possible

[Nicolas Pitre]

On Thu, 16 Nov 2017, Marc Gonzalez wrote:

> On 16/11/2017 17:08, Nicolas Pitre wrote:
> 
> > On Thu, 16 Nov 2017, Marc Gonzalez wrote:
> > 
> >> On 16/11/2017 16:36, Russell King - ARM Linux wrote:
> >>> On Thu, Nov 16, 2017 at 04:26:51PM +0100, Marc Gonzalez wrote:
> >>>> On 15/11/2017 14:13, Russell King - ARM Linux wrote:
> >>>>
> >>>>> udelay() needs to offer a consistent interface so that drivers know
> >>>>> what to expect no matter what the implementation is.  Making one
> >>>>> implementation conform to your ideas while leaving the other
> >>>>> implementations with other expectations is a recipe for bugs.
> >>>>>
> >>>>> If you really want to do this, fix the loops_per_jiffy implementation
> >>>>> as well so that the consistency is maintained.
> >>>>
> >>>> Hello Russell,
> >>>>
> >>>> It seems to me that, when using DFS, there's a serious issue with loop-based
> >>>> delays. (IIRC, it was you who pointed this out a few years ago.)
> >>>>
> >>>> If I'm reading arch/arm/kernel/smp.c correctly, loops_per_jiffy is scaled
> >>>> when the frequency changes.
> >>>>
> >>>> But arch/arm/lib/delay-loop.S starts by loading the current value of
> >>>> loops_per_jiffy, computes the number of times to loop, and then loops.
> >>>> If the frequency increases when the core is in __loop_delay, the
> >>>> delay will be much shorter than requested.
> >>>>
> >>>> Is this a correct assessment of the situation?
> >>>
> >>> Absolutely correct, and it's something that people are aware of, and
> >>> have already catered for while writing their drivers.
> >>
> >> In their cpufreq driver?
> >> In "real" device drivers that happen to use delays?
> >>
> >> On my system, the CPU frequency may ramp up from 120 MHz to 1.2 GHz.
> >> If the frequency increases at the beginning of __loop_delay, udelay(100)
> >> would spin only 10 microseconds. This is likely to cause issues in
> >> any driver using udelay.
> >>
> >> How does one cater for that?
> > 
> > You make sure your delays are based on a stable hardware timer.
> > Most platforms nowadays should have a suitable timer source.
> 
> So you propose fixing loop-based delays by using clock-based delays,
> is that correct? (That is indeed what I did on my platform.)
> 
> Russell stated that there are platforms using loop-based delays with
> cpufreq enabled. I'm asking how they manage the brokenness.

Look at cpufreq_callback() in arch/arm/kernel/smp.c.


Nicolas