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

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Message-Id: <dbd5558a-90d9-404c-ae98-a8c04cdad08a@app.fastmail.com>
Date: Thu, 30 Oct 2025 09:33:39 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Dan Carpenter" <dan.carpenter@...aro.org>
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 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?

> If we left out the "regs" property it wouldn't be created automatically
> but syscon_regmap_lookup_by_phandle() will return -EINVAL and probe would
> fail.  It needs to be -EPROBE_DEFER so the probe tries again after the
> regmap is registered.  We'd need to add a check like this (untested):

Right, this is exactly what I had in mind. With a new kernel and old
dtb, this would not change anything, while an old kernel but new dtb
would run into a different bug and fail to probe instead of using the
wrong device. I think both cases are fine.

     Arnd