Message-ID: <20251112142632.GA1610836-robh@kernel.org>
Date: Wed, 12 Nov 2025 08:26:32 -0600
From: Rob Herring <robh@...nel.org>
To: Herve Codina <herve.codina@...tlin.com>
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Subject: Re: [PATCH v4 05/29] dt-bindings: bus: Add simple-platform-bus

On Fri, Oct 31, 2025 at 04:20:04PM +0100, Herve Codina wrote:
> Hi Rob,
> 
> On Thu, 30 Oct 2025 09:14:48 -0500
> Rob Herring <robh@...nel.org> wrote:
> 
> > On Wed, Oct 15, 2025 at 09:13:52AM +0200, Herve Codina wrote:
> > > A Simple Platform Bus is a transparent bus that doesn't need a specific
> > > driver to perform operations at bus level.
> > > 
> > > Similar to simple-bus, a Simple Platform Bus allows to automatically
> > > instantiate devices connected to this bus.
> > > 
> > > Those devices are instantiated only by the Simple Platform Bus probe
> > > function itself.  
> > 
> > Don't let Greg see this... :)
> > 
> > I can't say I'm a fan either. "Platform bus" is a kernel thing, and the 
> > distinction here between the 2 compatibles is certainly a kernel thing.
> > 
> > I think this needs to be solved within the kernel.
> 
> I fully agree with that.
> 
> > 
> > What I previously said is define a list of compatibles to not 
> > instantiate the child devices. This would essentially be any case having 
> > a specific compatible and having its own driver. So if someone has 
> > 'compatible = "vendor,not-so-simple-bus", "simple-bus"', when and if 
> > they add a driver for "vendor,not-so-simple-bus", then they have to add 
> > the compatible to the list in the simple-pm-bus driver. I wouldn't 
> > expect this to be a large list. There's only a handful of cases where 
> > "simple-bus" has a more specific compatible. And only a few of those 
> > have a driver. A more general and complicated solution would be making 
> > linux handle 2 (or more) drivers matching a node and picking the driver 
> > with most specific match. That gets complicated with built-in vs. 
> > modules. I'm not sure we really need to solve that problem.
> 
> Right. Let discard the "more general and complicated solution" and focus
> on the list of compatible to avoid child devices instantiation.
> 
> Do you mean that, for "simple-bus" compatible we should:
>  - Remove the recursive device instantiation from of_platform_populate().

That may be a problem I hadn't considered. While we've solved most probe 
ordering issues, I think some may remain. Even when of_platform_populate() 
is called affects this. For example, I tried removing various arm32 
of_platform_.*populate() calls which run earlier than the default call, 
but that broke some platforms. (Looking at the list of remaining ones, I 
fixed the at91 pinctrl/gpio drivers, but never tried to remove the 
calls again.)

Maybe this can be restricted to cases which are not recursively created 
from the root node. Not sure how we detect that. Perhaps no OF_POPULATED 
flag on the parent node? Or we could just enable this for OF_DYNAMIC 
nodes? That should be sufficient for your usecase.

I would like to solve this more generally though. So we could try it in 
kernelci and/or linux-next and see what happens.

>  - In simple-bus probe(), check the device we probe against the
>    'no_instantiate_children' list
>       - If it matches, do not instantiate chidren
>       - If it doesn't match instantiate children

Right.

Rob

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