linux-kernel - Re: [PATCH v4 1/1] device property: Allow error pointer to be passed to fwnode APIs

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Message-ID: <CAJZ5v0hbY8XCC-DfkoPFe15awV_FOpq91pUZvmZ9JrYi1QBMEg@mail.gmail.com>
Date:   Tue, 15 Mar 2022 12:16:01 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Michael Walle <michael@...le.cc>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Daniel Scally <djrscally@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nuno Sá <nuno.sa@...log.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Russell King <linux@...linux.org.uk>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>
Subject: Re: [PATCH v4 1/1] device property: Allow error pointer to be passed
 to fwnode APIs

On Mon, Mar 14, 2022 at 8:51 PM Michael Walle <michael@...le.cc> wrote:
>
> Hi Andy,
>
> > Some of the fwnode APIs might return an error pointer instead of NULL
> > or valid fwnode handle. The result of such API call may be considered
> > optional and hence the test for it is usually done in a form of
> >
> >       fwnode = fwnode_find_reference(...);
> >       if (IS_ERR(fwnode))
> >               ...error handling...
> >
> > Nevertheless the resulting fwnode may have bumped the reference count
> > and hence caller of the above API is obliged to call fwnode_handle_put().
> > Since fwnode may be not valid either as NULL or error pointer the check
> > has to be performed there. This approach uglifies the code and adds
> > a point of making a mistake, i.e. forgetting about error point case.
> >
> > To prevent this, allow an error pointer to be passed to the fwnode APIs.
> >
> > Fixes: 83b34afb6b79 ("device property: Introduce fwnode_find_reference()")
> > Reported-by: Nuno Sá <nuno.sa@...log.com>
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > Tested-by: Nuno Sá <nuno.sa@...log.com>
> > Reviewed-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > Acked-by: Nuno Sá <nuno.sa@...log.com>
>
> This breaks SFP/phylink (using the lan966x switch) on my board. See below
> for more details.

I'm dropping this commit for the time being.

> [..]
>
> > @@ -480,15 +485,16 @@ int fwnode_property_get_reference_args(const struct fwnode_handle *fwnode,
> >  {
> >       int ret;
> >
> > +     if (IS_ERR_OR_NULL(fwnode))
> > +             return -ENOENT;
> > +
> >       ret = fwnode_call_int_op(fwnode, get_reference_args, prop, nargs_prop,
> >                                nargs, index, args);
> > +     if (ret == 0)
>
> Should this be "if (ret == 0 || IS_ERR_OR_NULL(fwnode->secondary))" ?
>
> > +             return ret;
> >
> > -     if (ret < 0 && !IS_ERR_OR_NULL(fwnode) &&
> > -         !IS_ERR_OR_NULL(fwnode->secondary))
> > -             ret = fwnode_call_int_op(fwnode->secondary, get_reference_args,
> > -                                      prop, nargs_prop, nargs, index, args);
> > -
> > -     return ret;
> > +     return fwnode_call_int_op(fwnode->secondary, get_reference_args, prop, nargs_prop,
> > +                               nargs, index, args);
> >  }
> >  EXPORT_SYMBOL_GPL(fwnode_property_get_reference_args);
>
> What happens before this patch is that sfp_bus_find_fwnode() will call
> fwnode_property_get_reference_args() and the first calls return -ENOENT
> which sfp_bus_find_fwnode() will handle in a special way. After your
> patch, -EINVAL is returned, because fwnode_call_int_op() on
> fwnode->secondary is always called regardless of the return value of
> the original fwnode.
>
> -michael