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

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Message-ID: <870aeb80-bbc5-52ed-cca2-3e762e27bfe2@sigmadesigns.com>
Date:   Thu, 16 Nov 2017 17:51:05 +0100
From:   Marc Gonzalez <marc_gonzalez@...madesigns.com>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
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 16/11/2017 17:47, Nicolas Pitre wrote:

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

Are you pointing at the scaling of loops_per_jiffy done in that function?

As I wrote earlier:

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.