Re: [PATCH v5 04/11] pwm: max7360: Add MAX7360 PWM support

[Uwe Kleine-König]

On Wed, Mar 26, 2025 at 05:49:07PM +0200, Andy Shevchenko wrote:
> On Wed, Mar 26, 2025 at 03:44:28PM +0100, Mathieu Dubois-Briand wrote:
> > On Tue Mar 25, 2025 at 4:56 PM CET, Andy Shevchenko wrote:
> > > On Tue, Mar 25, 2025 at 03:37:29PM +0100, Mathieu Dubois-Briand wrote:
> > > > On Thu Mar 20, 2025 at 11:48 AM CET, Andy Shevchenko wrote:
> > > > > On Thu, Mar 20, 2025 at 08:50:00AM +0100, Uwe Kleine-König wrote:
> > > > > > On Wed, Mar 19, 2025 at 01:18:50PM +0200, Andy Shevchenko wrote:
> > > > > > > On Tue, Mar 18, 2025 at 05:26:20PM +0100, mathieu.dubois-briand@xxxxxxxxxxx wrote:
> 
> ...
> 
> > > > > > > > +	chip = devm_pwmchip_alloc(dev->parent, MAX7360_NUM_PWMS, 0);
> > > > > > > 
> > > > > > > This is quite worrying. The devm_ to parent makes a lot of assumptions that may
> > > > > > > not be realised. If you really need this, it has to have a very good comment
> > > > > > > explaining why and object lifetimes.
> > > > > > 
> > > > > > Pretty sure this is broken. This results for example in the device link
> > > > > > being created on the parent. So if the pwm devices goes away a consumer
> > > > > > might not notice (at least in the usual way). I guess this was done to
> > > > > > ensure that #pwm-cells is parsed from the right dt node? If so, that
> > > > > > needs a different adaption. That will probably involve calling
> > > > > > device_set_of_node_from_dev().
> > > > >
> > > > > It's an MFD based driver, and MFD core cares about propagating fwnode by
> > > > > default. I believe it should just work if we drop that '->parent' part.
> > > > 
> > > > Are you sure about that?
> > >
> > > Yes and no. If your DT looks like (pseudo code as I don't know
> > > DTS syntax by heart):
> > >
> > > 	device: {
> > > 		parent-property = value;
> > > 		child0:
> > > 			...
> > > 		child1:
> > > 			...
> > > 	}
> > >
> > > the parent-property value is automatically accessible via fwnode API,
> > > but I don't know what will happen to the cases when each of the children
> > > has its own compatible string. This might be your case, but again,
> > > I'm not an expert in DT.
> > >
> > 
> > On my side:
> > - Some MFD child do have a child node in the device tree, with an
> >   associated compatible value. No problem for these, they do get correct
> >   of_node/fwnode values pointing on the child device tree node.
> > - Some MFD child do not have any node in the device tree, and for these,
> >   they have to use properties from the parent (MFD) device tree node.
> >   And here we do have some problems.
> > 
> > > > On my side it does not work if I just drop the '->parent', this is why I
> > > > ended whit this (bad) pattern.
> > >
> > > > Now it does work if I do call device_set_of_node_from_dev() manually,
> > >
> > > AFAICT, this is wrong API to be called in the children. Are you talking about
> > > parent code?
> > >
> > 
> > I believe I cannot do it in the parent code, as I would need to do it
> > after the call to devm_mfd_add_devices(), and so it might happen after
> > the probe. I still tried to see how it behaved, and it looks like PWM
> > core really did not expect to get an of_node assigned to the device
> > after adding the PWM device.
> > 
> > So either I can do something in MFD core or in sub devices probe(), or I
> > need to come with a different way to do things.
> > 
> > > > so it's definitely better. But I believe the MFD core is not propagating
> > > > OF data, and I did not find where it would do that in the code. Yet it
> > > > does something like this for ACPI in mfd_acpi_add_device(). Or maybe we
> > > > do something bad in our MFD driver?
> > >
> > > ...or MFD needs something to have... Dunno.
> > 
> > I have something working with a very simple change in mfd-core.c, but
> > I'm really not confident it won't break anything else. I wish I could
> > get some insights from an MFD expert.
> > 
> > @@ -210,6 +210,8 @@ static int mfd_add_device(struct device *parent, int id,
> >                 if (!pdev->dev.of_node)
> >                         pr_warn("%s: Failed to locate of_node [id: %d]\n",
> >                                 cell->name, platform_id);
> > +       } else if (IS_ENABLED(CONFIG_OF) && parent->of_node) {
> > +               device_set_of_node_from_dev(&pdev->dev, parent);
> 
> The use of this API is inappropriate here AFAICT. It drops the parent refcount
> and on the second call to it you will have a warning from refcount library.

device_set_of_node_from_dev() does:

	of_node_put(pdev->dev->of_node);
	pdev->dev->of_node = of_node_get(parent->of_node);
	pdev->dev->of_node_reused = true;

Note that pdev isn't the platform device associated with the parent
device but the just allocated one representing the subdevice so
pdev->dev->of_node is NULL and the parent refcount isn't dropped.

But I'm unsure if this is the right place to call it or if
device_set_node() is indeed enough (also I wonder if
device_set_of_node_from_dev() should care for fwnode). I'll keep that
question for someone else.

Best regards
Uwe

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