linux-kernel - Re: [PATCH 0/2] mfd: syscon: introduce no-auto-mmio DT property

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Message-ID: <aQMmq4BfsNzAEY78@stanley.mountain>
Date: Thu, 30 Oct 2025 11:49:47 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Lee Jones <lee@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
	devicetree@...r.kernel.org,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	linux-kernel@...r.kernel.org, Rob Herring <robh@...nel.org>,
	André Draszik <andre.draszik@...aro.org>,
	Peter Griffin <peter.griffin@...aro.org>
Subject: Re: [PATCH 0/2] mfd: syscon: introduce no-auto-mmio DT property

On Thu, Oct 30, 2025 at 09:33:39AM +0100, Arnd Bergmann wrote:
> On Thu, Oct 30, 2025, at 08:33, Dan Carpenter wrote:
> > On Wed, Oct 29, 2025 at 08:43:33PM +0100, Arnd Bergmann wrote:
> >> On Wed, Oct 29, 2025, at 18:27, Dan Carpenter wrote:
> >> > Most syscons are accessed via MMMIO and created automatically.  But one
> >> > example of a syscon that isn't is in drivers/soc/samsung/exynos-pmu.c
> >> > where the syscon can only be accessed via the secure partition.  We are
> >> > looking at upstreaming a different driver where the syscon will be
> >> > accessed via SCMI.
> >> >
> >> > Normally, syscons are accessed by doing something like
> >> > syscon_regmap_lookup_by_phandle_args() but that function will
> >> > automatically create an MMIO syscon if one hasn't been registered.  So
> >> > the ordering becomes a problem.  The exynos-pmu.c driver solves this
> >> > but it's a bit awkward and it would be even trickier if there were
> >> > several drivers accessing the same syscon.
> >> 
> >> What would happen on the current exynos platform if we just take away
> >> the 'regs' property? I would hope that we can avoid encoding what
> >> is essentially operating system policy in that driver and instead
> >> just describe it as a device that expects to be implemented by
> >> firmware and doesn't need registers?
> >
> > Exynos solves this because they only have one phandle so when they parse
> > it, that's when then they create the syscon.  If you had multiple drivers
> > accessing the same syscon then that doesn't work.
> 
> I'm not following the logic here.  Do you mean that they avoid the
> issue today by ensuring that the regmap is always probed before
> its only user, or do you mean something else?

Yes.  That's what I mean.

regards,
dan carpenter