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

[Russell King - ARM Linux]

On Thu, Nov 16, 2017 at 05:42:36PM +0100, Marc Gonzalez wrote:
> On 16/11/2017 17:32, Russell King - ARM Linux wrote:
> > On Thu, Nov 16, 2017 at 05:26:32PM +0100, 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.
> > 
> > Quite simply, they don't have such a wide range of frequencies that can
> > be selected.  They're on the order of 4x.  For example, the original
> > platform that cpufreq was developed on, a StrongARM-1110 board, can
> > practically range from 221MHz down to 59MHz.
> 
> Requesting 100 µs and spinning only 25 µs is still a problem,
> don't you agree?

Which is why, as I've said *many* times already, that drivers are written
with leaway on the delays.

I get the impression that we're just going around in circles, and what
you're trying to do is to get me to agree with your point of view.
That's not going to happen, because I know the history over about the
last /24/ years of kernel development (which is how long I've been
involved with the kernel.)  That's almost a quarter of a century!

I know how things were done years ago (which is relevant because we
still have support in the kernel for these systems), and I also know the
history of facilities like cpufreq - I was the one who took the work
that Erik Mouw and others involved with the LART project, and turned it
into something a little more generic.  The idea of dynamically scaling
the CPU frequency on ARM SoCs was something that the SoC manufacturer
had not even considered - it was innovative.

I know that udelay() can return short delays when used in a kernel with
cpufreq enabled, and I also know that's almost an impossible problem to
solve without going to a timer-based delay.

So, when you think that sending an email about a udelay() that can be
10x shorter might be somehow new information, and might convince people
that there's a problem, I'm afraid that it isn't really new information.
The SA1110 cpufreq driver is dated 2001, and carries my copyright, and
has the ability to make udelay()s 4x shorter or 4x longer depending on
the direction of change.

We've discussed solutions in the past (probably 10 years ago) about
this, and what can be done, and the conclusion to that was, as Nicolas
has said, to switch to using a timer-based delay mechanism where
possible.  Where this is not possible, the platform is stuck with the
loops based delays, and their inherent variability and inaccuracy.

These platforms have been tested with such a setup over many years.
They work even with udelay() having this behaviour, because it's a
known issue and drivers cater for it in ways that I've already covered
in my many previous emails to you.

These issues are known.  They've been known for the last 15 odd years.

> > BTW, your example above is incorrect.
> 
> A 10x increase in frequency causes a request of 100 µs to spin
> only 10 µs, as written above.

Right, sorry, misread.

-- 
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