linux-kernel - Re: [PATCH 1/2] regmap: teach regmap to use raw spinlocks if requested in the config

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Message-ID: <20210830104909.aknshexcge6s7z27@skbuf>
Date:   Mon, 30 Aug 2021 10:49:10 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Mark Brown <broonie@...nel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Lee Jones <lee.jones@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Marc Zyngier <maz@...nel.org>,
        "Z.Q. Hou" <zhiqiang.hou@....com>, Biwen Li <biwen.li@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] regmap: teach regmap to use raw spinlocks if
 requested in the config

On Fri, Aug 27, 2021 at 09:59:56PM +0200, Thomas Gleixner wrote:
> On Fri, Aug 27 2021 at 16:12, Vladimir Oltean wrote:
> > On Fri, Aug 27, 2021 at 01:01:35AM +0200, Thomas Gleixner wrote:
> >> Even for the case where the regmap is not dealing with irq chips it does
> >> not make any sense to protect low level operations on shared register
> >> with a regular spinlock. I might be missing something though...
> >
> > Mark, any comments?
> >
> > Generally it is said that misusing raw spinlocks has detrimential
> > performance upon the real-time aspects of the system, and I don't really
> > have a good feeling for what constitutes misuse vs what is truly justified
> > (in fact I did start the thread with "apologies for my novice level of
> > understanding").
> >
> > On the other hand, while it does seem a bit too much overhead for
> > sequences of MMIO reads/writes to be able to be preempted, it doesn't
> > sound like it would break something either, so...
>
> The question is how long those sequences are.
>
> If it's just a pair or so then the raw spinlock protection has
> definitely a smaller worst case than the sleeping spinlock in the
> contended case.
>
> OTOH, if regmap operations consist of several dozens of MMIO accesses,
> then the preempt disabled region might be quite long.
>
> I'm not familiar enough with regmaps to make a judgement here.

I think "how long are the read/write regmap sequences" is outside of
regmap's control, but rather a matter of usage. This would point towards
the current solution, where users select whether to use raw spinlocks or
not, being the preferable one.