Date: Wed, 29 May 2024 18:17:14 +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 06:00:19PM +0300, Andy Shevchenko wrote:
> On Wed, May 29, 2024 at 5:35 PM Laurent Pinchart wrote:
> > 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 is something I need to refresh in my memory. Any pointers (the
> links to the exact code lines are also okay) where I can find the
> proof of what you are saying. (It's not that I untrust you, it's just
> that I take my time on studying it.)

I wish this was documented, I could then point you to a nice text that
explains it all :-) My understanding comes from reading
platform_device_release(), and from failing to find other of_node_put()
calls that would be relevant to this.

> > > > 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
> 
> s/very/a/ (sorry for the confusion)
> 
> > > 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.
> 
> What you are doing is sharing the parent node with the child, but at
> the same time you bump the parent's reference count. As this is a
> child of MFD I don't think you need this as MFD already takes care of
> it via parent -> child natural dependencies. Is it incorrect
> understanding?

It is true that the parent device will not be destroyed as long as the
child device exists. The parent device's of_node will thus be kept
around by the reference that the parent device holds on it. However, if
I set dev.of_node for the child device without acquiring a reference, we
will have dev.of_node pointing to the same device_node, with a single
reference to it. This means that when the child device is destroyed and
platform_device_release() is called for it, of_node_put() will release
that reference, and the parent dev.of_node will point to a device_node
without holding a reference. Then, when the parent device, which is an
i2c_client, is unregistered, the call to i2c_unregister_device() will
call of_node_put(), releasing a reference we don't have.

The order of i2c_unregister_device() and platform_device_release() may
be swapped in practice, I haven't double-checked, but the reasoning
still holds. We have two functions that expect dev.of_node to hold a
reference, so both dev.of_node need to hold a reference.

Another way to handle the problem would be to borrow the parent's
reference in the probe function of the child, and set dev.of_node to
NULL manually in the child .remove() function. This will ensure that
platform_device_release(), which is called after .remove() (not
necessarily directly after, but certainly not before), will not attempt
to incorrectly release a reference on dev.of_node. This will however not
be safe if i2c_unregister_device() is called before the child .remove(),
as the child dev.of_node would then for some amount of time point to a
device_node without a reference.

TL;DR: Using device_set_of_node_from_dev() seems the safest option,
especially given that it was designed for this purpose.

-- 
Regards,

Laurent Pinchart

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