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

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Date:   Wed, 1 Sep 2021 17:05:55 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Vladimir Oltean <vladimir.oltean@....com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Lee Jones <lee.jones@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Marc Zyngier <maz@...nel.org>,
        Hou Zhiqiang <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
Subject: Re: [PATCH 1/2] regmap: teach regmap to use raw spinlocks if
 requested in the config

On Mon, Aug 30, 2021 at 04:16:04PM +0200, Thomas Gleixner wrote:
> On Mon, Aug 30 2021 at 13:19, Mark Brown wrote:

> > That probably does make sense, I think we're just using regular
> > spinlocks for spinlocks mainly because they're the default rather
> > than because anyone put huge amounts of thought into it.  IIRC
> > the first users were using spinlocks for their locking when they
> > were converted.

> So if the actual spinlock protected operations are not doing any other
> business than accessing preallocated cache memory and a few MMIO
> operations then converting them to raw spinlocks should have no real
> impact on RT.

I think Vladimir's point that something might try to use one of the APIs
that can do multiple register writes atomically to generate a very long
register write sequence is valid here.  It's far from the common case
but it'd be hard to audit, it's going to be a lot easier to handle going
to raw spinlocks in the cases where it's specifically needed than to
keep on top of ensuring that none of the users are causing issues or
start causing issues in the future.  This does make me feel it's a bit
safer to leave the default the way it is since if you get it wrong then
lockdep will tend to notice very quickly while it's less likely that
we'd get tooling spotting issues the other way around.

> One way to do that is obviously starting with the patch from Vladimir
> and then convert them one by one, so the assumption that they are not
> doing anything nasty (in the RT sense) can be validated.

Vladimir's patch is in Linus' tree now so users that can safely do so
can start using raw spinlocks.

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