linux-kernel - Re: [PATCH v2 1/6] device property: Add functions for accessing node's parents

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Date:   Thu, 28 Mar 2019 13:13:08 +0200
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v2 1/6] device property: Add functions for accessing
 node's parents

Hi Rafael,

On Thu, Mar 28, 2019 at 10:38:01AM +0100, Rafael J. Wysocki wrote:
> On Wed, Mar 27, 2019 at 3:20 PM Sakari Ailus
> <sakari.ailus@...ux.intel.com> wrote:
> >
> > Hi Petr,
> >
> > On Wed, Mar 27, 2019 at 01:26:25PM +0100, Petr Mladek wrote:
> > > On Tue 2019-03-26 14:41:01, Sakari Ailus wrote:
> > > > Add two convenience functions for accessing node's parents:
> > > >
> > > > fwnode_count_parents() returns the number of parent nodes a given node
> > > > has. fwnode_get_nth_parent() returns node's parent at a given distance
> > > > from the node itself.
> > > >
> > > > Also reorder fwnode_get_parent() in property.c according to the same order
> > > > as in property.h.
> > > >
> > > > diff --git a/drivers/base/property.c b/drivers/base/property.c
> > > > index 8b91ab380d14..61eb6ceacc3f 100644
> > > > --- a/drivers/base/property.c
> > > > +++ b/drivers/base/property.c
> > > > @@ -554,17 +567,49 @@ struct fwnode_handle *fwnode_get_next_parent(struct fwnode_handle *fwnode)
> > > >  EXPORT_SYMBOL_GPL(fwnode_get_next_parent);
> > > >
> > > >  /**
> > > > - * fwnode_get_parent - Return parent firwmare node
> > > > - * @fwnode: Firmware whose parent is retrieved
> > > > + * fwnode_count_parents - Return the number of parents a node has
> > > > + * @fwnode: The node the parents of which are to be counted
> > > >   *
> > > > - * Return parent firmware node of the given node if possible or %NULL if no
> > > > - * parent was available.
> > > > + * Returns the number of parents a node has.
> > > >   */
> > > > -struct fwnode_handle *fwnode_get_parent(const struct fwnode_handle *fwnode)
> > > > +unsigned int fwnode_count_parents(struct fwnode_handle *fwnode)
> > > >  {
> > > > -   return fwnode_call_ptr_op(fwnode, get_parent);
> > > > +   unsigned int count;
> > > > +
> > > > +   fwnode_handle_get(fwnode);
> > > > +
> > > > +   for (count = 0; fwnode; count++)
> > > > +           fwnode = fwnode_get_next_parent(fwnode);
> > >
> > > Is it guaranteed that all parents stay when
> > > fwnode_get_next_parent() releases the reference count
> > > for each counted member?
> >
> > fwnode_get_next_parent() only releases the child node after it has acquired
> > the parent. The only implementation with refcounting for single nodes is
> > actually OF.
> >
> > >
> > > > +
> > > > +   return count - 1;
> > >
> > > We could start counting from count = -1;
> >
> > We could, but then count would need to be made signed (unless overflowing
> > is preferred instead).
> 
> What if there are no parents?
> 
> Or is it guaranteed that there always will be at least one?

If there are no parents, the function will return 0 --- the node itself is
counted in the loop, too, hence the need to decrement by one.

-- 
Regards,

Sakari Ailus
sakari.ailus@...ux.intel.com