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

[Andy Shevchenko <andriy.shevchenko@intel.com>]

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@bootlin.com 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.

It should be as simple as device_set_node().

>         }

With that, the conditional becomes

	} else if (is_of_node(fwnode)) {
		device_set_node(&pdev->dev, fwnode);
	}

where fwnode is something like

	struct fwnode_handle *fwnode = dev_fwnode(parent);

-- 
With Best Regards,
Andy Shevchenko