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

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Date:   Thu, 16 Nov 2017 11:47:20 -0500 (EST)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Marc Gonzalez <marc_gonzalez@...madesigns.com>
cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Jonathan Austin <jonathan.austin@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Kevin Hilman <khilman@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Mason <slash.tmp@...e.fr>
Subject: Re: [RFC] Improving udelay/ndelay on platforms where that is
 possible

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