linux-kernel - Re: [PATCH v2 1/3] software node: Introduce device_add_software

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Date:   Thu, 14 Jan 2021 16:00:40 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Daniel Scally <djrscally@...il.com>,
        Felipe Balbi <balbi@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2 1/3] software node: Introduce
 device_add_software_node()

On Wed, Jan 13, 2021 at 05:58:12PM +0200, Andy Shevchenko wrote:
> On Wed, Jan 13, 2021 at 05:55:04PM +0200, Andy Shevchenko wrote:
> > On Wed, Jan 13, 2021 at 12:40:03AM +0000, Daniel Scally wrote:
> > > On 11/01/2021 14:10, Heikki Krogerus wrote:
> > 
> > ...
> > 
> > > > +/**
> > > > + * device_remove_software_node - Remove device's software node
> > > > + * @dev: The device with the software node.
> > > > + *
> > > > + * This function will unregister the software node of @dev.
> > > > + */
> > > > +void device_remove_software_node(struct device *dev)
> > > > +{
> > > > +	struct swnode *swnode;
> > > > +
> > > > +	swnode = dev_to_swnode(dev);
> > > > +	if (!swnode)
> > > > +		return;
> > > > +
> > > > +	kobject_put(&swnode->kobj);
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(device_remove_software_node);
> > > 
> > > I wonder if this also ought to set dev_fwnode(dev)->secondary back to
> > > ERR_PTR(-ENODEV)?
> > 
> > Looking more into this code I think we need to call
> > 
> > 	set_secondary_fwnode(dev, NULL);
> > 
> > among these lines.
> > 
> > The real problem is that set_primary_fwnode() and set_secondary_fwnode() have
> > no reference counting. If we have a chain ->primary->secondary->-ENODEV is
> > being used somewhere we can't tell from here.
> > 
> > So, in practice it means that we lack of some kind of primary node to increment
> > reference count of the secondary node when the latter is chained to the given
> > primary. But it makes things too complicated. Any other options for shared
> > primary-secondary chain? Standalone primary along with standalone (exclusive)
> > secondary doesn't need this AFAICS. Perhaps a flag to primary like shared /
> > exclusive that will prevent breaking the chain in set_secondary_fwnode()?
> 
> Or maybe I imagined only theoretical cases and we have no such issue?

I think we should really start looking into the possibility of
removing the whole secondary coupling, because that is the thing that
is crippling us.

Br,

-- 
heikki