Re: [PATCH v3 05/28] bus: simple-pm-bus: Populate child nodes at probe

[Herve Codina <herve.codina@bootlin.com>]

Hi Rob,

On Mon, 14 Jul 2025 12:44:22 -0500
Rob Herring <robh@kernel.org> wrote:

> On Fri, Jul 4, 2025 at 3:57 AM Herve Codina <herve.codina@bootlin.com> wrote:
> >
> > Hi Rob,
> >
> > On Thu, 3 Jul 2025 09:33:02 +0200
> > Herve Codina <herve.codina@bootlin.com> wrote:
> >  
> > > Hi Rob,
> > >
> > > On Fri, 27 Jun 2025 10:52:00 -0500
> > > Rob Herring <robh@kernel.org> wrote:
> > >  
> > > > On Fri, Jun 13, 2025 at 03:47:45PM +0200, Herve Codina wrote:  
> > > > > The simple-pm-bus driver handles several simple busses. When it is used
> > > > > with busses other than a compatible "simple-pm-bus", it doesn't populate
> > > > > its child devices during its probe.
> > > > >
> > > > > This confuses fw_devlink and results in wrong or missing devlinks.
> > > > >
> > > > > Once a driver is bound to a device and the probe() has been called,
> > > > > device_links_driver_bound() is called.
> > > > >
> > > > > This function performs operation based on the following assumption:
> > > > >     If a child firmware node of the bound device is not added as a
> > > > >     device, it will never be added.
> > > > >
> > > > > Among operations done on fw_devlinks of those "never be added" devices,
> > > > > device_links_driver_bound() changes their supplier.
> > > > >
> > > > > With devices attached to a simple-bus compatible device, this change
> > > > > leads to wrong devlinks where supplier of devices points to the device
> > > > > parent (i.e. simple-bus compatible device) instead of the device itself
> > > > > (i.e. simple-bus child).
> > > > >
> > > > > When the device attached to the simple-bus is removed, because devlinks
> > > > > are not correct, its consumers are not removed first.
> > > > >
> > > > > In order to have correct devlinks created, make the simple-pm-bus driver
> > > > > compliant with the devlink assumption and create its child devices
> > > > > during its probe.  
> > > >
> > > > IIRC, skipping child nodes was because there were problems with
> > > > letting the driver handle 'simple-bus'. How does this avoid that now?  
> > >
> > > I don't know about the specific issues related to those problems. Do you
> > > have some pointers about them?
> > >  
> > > >
> > > > The root of_platform_populate() that created the simple-bus device that
> > > > gets us to the probe here will continue descending into child nodes.
> > > > Meanwhile, the probe here is also descending into those same child
> > > > nodes. Best case, that's just redundant. Worst case, won't you still
> > > > have the same problem if the first of_platform_populate() creates the
> > > > devices first?
> > > >  
> > >
> > > Maybe we could simply avoid of_platform_populate() to be recursive when a
> > > device populate by of_platform_populate() is one of devices handled by
> > > the simple-bus driver and let the simple-bus driver do the job.
> > >
> > > of_platform_populate will handle the first level. It will populate children
> > > of the node given to of_platform_populate() and the children of those
> > > children will be populate by the simple-bus driver.
> > >
> > > I could try a modification in that way. Do you think it could be a correct
> > > solution?
> > >  
> >
> > I have started to look at this solution and it's going to be more complex
> > than than I thought.
> >
> > Many MFD drivers uses a compatible of this kind (the same exist for bus
> > driver with "simple-bus"):
> >   compatible = "foo,bar", "simple-mfd";
> >
> > Usually the last compatible string ("simple-mfd" here) is a last fallback
> > and the first string is the more specific one.
> >
> > In the problematic case, "foo,bar" has a specific driver and the driver
> > performs some operations at probe() but doesn't call of_platform_populate()
> > and relies on the core to do the device creations (recursively) based on
> > the "simple,mfd" string present in the compatible property.
> >
> > Some other calls of_platform_populate() in they probe (which I think is
> > correct) and in that case, the child device creation can be done at two
> > location: specific driver probe() and core.
> >
> > You pointed out that the core could create devices before the specific
> > driver is probed. In that case, some of existing drivers calling
> > of_platform_populate() are going to have issues.
> >
> > I can try to modify existing MFD and bus drivers (compatible fallback to
> > simple-mfd, simple-bus, ...) in order to have them call of_platform_populate()
> > in they probe() and after all problematic drivers are converted, the
> > recursive creation of devices done in the core could be removed.  
> 
> The problem is how does a bus driver know if there is a specific MFD
> driver or not? It doesn't. The MFD driver could be a module and loaded
> any time later. We'd really need some sort of unbind the generic
> driver and re-bind to a more specific driver when and if that driver
> appears. We could perhaps have a list of devices with a driver because
> in theory that should be a short list as the (broken) promise of
> simple-mfd is the child nodes have no dependency on the parent node
> which implies the parent doesn't have a driver. The specific
> compatible is there in case that assumption turns out wrong.
> 

Hum, I see.

In my use case, I don't use MFD drivers but only simple-bus compatible.
I think your point is also relevant with simple-bus. Indeed how does a
parent bus driver know if there is a specific bus driver that handles
the child simple-bus compatible one in case of 'simple-bus' used as
fallback.

Related to your proposal related to the "list of devices with a driver",
what do you mean? I don't see how to set this kind of list. Can you give
me some pointers?

If I understood the discussion, the issue seems that 'simple-bus' can't
populate unconditionally children at his probe. The possible recursion
in creating devices done by of_platform_populate() should be kept and
'simple-bus' should rely on that.

The other solution that fixes my use case is to use an other compatible
string. Would you accept a new compatible string: "simple-platform-bus"?

In simple-pm-bus.c, this compatible would populate children at probe.
In fact, it will act the same way as 'simple-pm-bus' without looking at
clocks nor handling pm_runtime.

Best regards,
Hervé