linux-kernel - Re: [PATCH v2 3/4] gpio: adp5585: Add Analog Devices ADP5585 support

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Date: Wed, 29 May 2024 17:35:06 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-gpio@...r.kernel.org, linux-pwm@...r.kernel.org,
	Bartosz Golaszewski <brgl@...ev.pl>,
	Conor Dooley <conor+dt@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Lee Jones <lee@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Rob Herring <robh@...nel.org>,
	Uwe Kleine-König <ukleinek@...nel.org>,
	Haibo Chen <haibo.chen@....com>
Subject: Re: [PATCH v2 3/4] gpio: adp5585: Add Analog Devices ADP5585 support

On Wed, May 29, 2024 at 05:24:03PM +0300, Andy Shevchenko wrote:
> On Wed, May 29, 2024 at 12:48 PM Laurent Pinchart wrote:
> > On Wed, May 29, 2024 at 09:16:43AM +0300, Andy Shevchenko wrote:
> > > On Tue, May 28, 2024 at 11:20 PM Laurent Pinchart wrote:
> > > > On Tue, May 28, 2024 at 10:36:06PM +0300, Andy Shevchenko wrote:
> 
> ...
> 
> > > > > > +   device_set_of_node_from_dev(dev, dev->parent);
> > > > >
> > > > > Why not device_set_node()?
> > > >
> > > > Because device_set_of_node_from_dev() is meant for this exact use case,
> > > > where the same node is used for multiple devices. It also puts any
> > > > previous dev->of_node, ensuring proper refcounting when devices are
> > > > unbound and rebound, without being deleted.
> > >
> > > When will the refcount be dropped (in case of removal of this device)?
> > > Or you mean it shouldn't?
> >
> > Any refcount taken on the OF node needs to be dropped. The device core
> > only drops the refcount when the device is being deleted, not when
> > there's an unbind-rebind cycle without deletion of the device (as
> > happens for instance when the module is unloaded and reloaded).
> 
> Under "device" you meant the real hardware, as Linux device (instance
> of the struct device object) is being rebuilt AFAIK)?

I mean struct device. The driver core will drop the reference in
platform_device_release(), called when the last reference to the
platform device is released, just before freeing the platform_device
instance. This happens after the device is removed from the system (e.g.
hot-unplug), but not when a device is unbound from a driver and rebound
(e.g. module unload and reload).

> > This has
> > to be handled by the driver. device_set_of_node_from_dev() handles it.
> 
> But why do you need to keep a parent node reference bumped?
> Only very few drivers in the kernel use this API and I believe either
> nobody knows what they are doing and you are right, or you are doing
> something which is not needed.

I need to set the of_node and fwnode fields of struct device to enable
OF-based lookups of GPIOs and PWMs. The of_node field is meant to be
populated by the driver core when the device is created, with a
reference to the OF node. When populated directly by driver, this needs
to be taken into account, and drivers need to ensure the reference will
be released correctly. device_set_of_node_from_dev() is meant for that.

-- 
Regards,

Laurent Pinchart