linux-kernel - Re: [PATCH v2 1/5] device property: Keep dev_fwnode() and dev_fwnode

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Message-ID: <Yzb9nXSxvgJ+Mj6z@paasikivi.fi.intel.com>
Date:   Fri, 30 Sep 2022 14:30:53 +0000
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Prashant Malani <pmalani@...omium.org>,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, Daniel Scally <djrscally@...il.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v2 1/5] device property: Keep dev_fwnode() and
 dev_fwnode_const() separate

Hi Greg,

On Wed, Sep 28, 2022 at 01:05:20PM +0200, Greg Kroah-Hartman wrote:
> On Wed, Sep 28, 2022 at 01:57:42PM +0300, Andy Shevchenko wrote:
> > It's not fully correct to take a const parameter pointer to a struct
> > and return a non-const pointer to a member of that struct.
> > 
> > Instead, introduce a const version of the dev_fwnode() API which takes
> > and returns const pointers and use it where it's applicable.
> > 
> > Suggested-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > Fixes: aade55c86033 ("device property: Add const qualifier to device_get_match_data() parameter")
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > Acked-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> > Reviewed-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > ---
> >  drivers/base/property.c  | 11 +++++++++--
> >  include/linux/property.h |  3 ++-
> >  2 files changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/base/property.c b/drivers/base/property.c
> > index 4d6278a84868..699f1b115e0a 100644
> > --- a/drivers/base/property.c
> > +++ b/drivers/base/property.c
> > @@ -17,13 +17,20 @@
> >  #include <linux/property.h>
> >  #include <linux/phy.h>
> >  
> > -struct fwnode_handle *dev_fwnode(const struct device *dev)
> > +struct fwnode_handle *dev_fwnode(struct device *dev)
> >  {
> >  	return IS_ENABLED(CONFIG_OF) && dev->of_node ?
> >  		of_fwnode_handle(dev->of_node) : dev->fwnode;
> >  }
> >  EXPORT_SYMBOL_GPL(dev_fwnode);
> >  
> > +const struct fwnode_handle *dev_fwnode_const(const struct device *dev)
> > +{
> > +	return IS_ENABLED(CONFIG_OF) && dev->of_node ?
> > +		of_fwnode_handle(dev->of_node) : dev->fwnode;
> > +}
> > +EXPORT_SYMBOL_GPL(dev_fwnode_const);
> 
> Ick, no, this is a mess.
> 
> Either always return a const pointer, or don't.  Ideally always return a
> const pointer, so all we really need is:
> 
> const struct fwnode_handle *dev_fwnode(const struct device *dev);
> 
> right?
> 
> Yes, it will take some unwinding backwards to get there, but please do
> that instead of having 2 different functions where the parameter type is
> part of the function name.  This isn't the 1980's...

The problem with this approach is that sometimes non-const fwnode_handles
are needed. On OF, for instance, anything that has something to do with
refcounting requires this. Software nodes as well.

One option which I suggested earlier was to turn dev_fwnode() into a macro
and use C11 _Generic() to check whether the device is const or not.

Being able to turn struct device pointers const is certainly not worth
violating constness properties.

-- 
Regards,

Sakari Ailus