linux-kernel - Re: [PATCH 1/2] dt-bindings: mfd: syscon: introduce no-auto-mmio property for syscons

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Message-ID: <aQJhTbNJkezeipoc@stanley.mountain>
Date: Wed, 29 Oct 2025 21:47:41 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Conor Dooley <conor@...nel.org>
Cc: Lee Jones <lee@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: mfd: syscon: introduce no-auto-mmio
 property for syscons

On Wed, Oct 29, 2025 at 06:37:26PM +0000, Conor Dooley wrote:
> On Wed, Oct 29, 2025 at 08:41:51PM +0300, Dan Carpenter wrote:
> > On Wed, Oct 29, 2025 at 05:33:48PM +0000, Conor Dooley wrote:
> > > On Wed, Oct 29, 2025 at 08:27:05PM +0300, Dan Carpenter wrote:
> > > > Generally, syscons are created automatically and accessed direclty via
> > > > MMIO however sometimes syscons might only be accessible from the secure
> > > > partition or through SCMI etc.  Introduce the no-auto-mmio property to
> > > > tell the operating system that the syscon needs to be handled manually.
> > > 
> > > "System controller node represents a register region containing a set
> > > of miscellaneous registers."
> > > 
> > > If this isn't actually a register region, but is instead an interface
> > > provided by SCMI or whatever "secure partition" is (optee?), why is the
> > > syscon compatible being used for the device in the first place?
> > 
> > In the case that I'm looking at, it really is a syscon.  So right now
> > we're upstreaming it and it's an MMIO syscon.  Very straight forward.
> > But later, I guess, they want to have a new firmware which will only let
> > you access the same registers through SCMI.
> 
> When the programming model changes, the compatible should too, no?
> 

I wasn't planning on it.  I haven't been asked to upstream the SCMI
module but once my thinking was the transition would work like this.

Step 1: It would work as is with an MMIO syscon.
Step 2: We would upstream the SCMI driver which would provide an
        MMIO syscon as a fallback.  At that stage you would still get an
        MMIO yscon regardless of whether the phandle was parsed before
        or after the driver loaded.
Step 3: We would set the no-auto-mmio property so you have to use the
        driver and update the firmware so only the SCMI interface can
        be used.

regards,
dan carpenter