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From: Vladimir Oltean <email@example.com>
To: Thomas Gleixner <firstname.lastname@example.org>
Cc: Mark Brown <email@example.com>,
Rasmus Villemoes <firstname.lastname@example.org>,
Lee Jones <email@example.com>, Arnd Bergmann <firstname.lastname@example.org>,
Marc Zyngier <email@example.com>, "Z.Q. Hou" <firstname.lastname@example.org>,
Biwen Li <email@example.com>,
Greg Kroah-Hartman <firstname.lastname@example.org>,
"Rafael J. Wysocki" <email@example.com>,
Subject: Re: [PATCH 1/2] regmap: teach regmap to use raw spinlocks if requested in the config
Date: Mon, 30 Aug 2021 10:49:10 +0000 [thread overview]
Message-ID: <20210830104909.aknshexcge6s7z27@skbuf> (raw)
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.
next prev parent reply other threads:[~2021-08-30 10:49 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-25 20:50 [PATCH 0/2] Use raw spinlocks in the ls-extirq driver Vladimir Oltean
2021-08-25 20:50 ` [PATCH 1/2] regmap: teach regmap to use raw spinlocks if requested in the config Vladimir Oltean
2021-08-26 23:01 ` Thomas Gleixner
2021-08-27 16:12 ` Vladimir Oltean
2021-08-27 19:59 ` Thomas Gleixner
2021-08-30 10:49 ` Vladimir Oltean [this message]
2021-08-30 11:02 ` Rasmus Villemoes
2021-08-30 12:42 ` Mark Brown
2021-08-30 12:19 ` Mark Brown
2021-08-30 14:16 ` Thomas Gleixner
2021-09-01 16:05 ` Mark Brown
2021-09-02 8:35 ` Thomas Gleixner
2021-08-25 20:50 ` [PATCH 2/2] mfd: syscon: request a regmap with raw spinlocks for some devices Vladimir Oltean
2021-08-25 21:24 ` Arnd Bergmann
2021-08-25 22:00 ` Vladimir Oltean
2021-08-26 9:24 ` Arnd Bergmann
2021-08-26 13:57 ` Vladimir Oltean
2021-08-26 14:48 ` Arnd Bergmann
2021-09-06 9:18 ` Lee Jones
2021-08-26 12:51 ` (subset) [PATCH 0/2] Use raw spinlocks in the ls-extirq driver Mark Brown
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