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

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Date:   Thu, 16 Nov 2017 22:05:43 +0100
From:   Marc Gonzalez <marc_gonzalez@...madesigns.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Nicolas Pitre <nicolas.pitre@...aro.org>,
        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 18:05, Russell King - ARM Linux wrote:

> On Thu, Nov 16, 2017 at 05:42:36PM +0100, Marc Gonzalez wrote:
>
>> 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.

A delay 75% too short is possible. Roger that.

> 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.

So you've known for umpteen years that fixing loop-based delays is
intractable, yet you 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.

In other words, "I'll consider your patch as soon as Hell freezes over".

Roger that. I'll drop the subject then.