Re: [PATCH v2 1/5] device property: Keep dev_fwnode() and dev_fwnode_const() separate

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Sep 30, 2022 at 02:30:53PM +0000, Sakari Ailus wrote:
> 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@linux.intel.com>
> > > Fixes: aade55c86033 ("device property: Add const qualifier to device_get_match_data() parameter")
> > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > > Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> > > Reviewed-by: Sakari Ailus <sakari.ailus@linux.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.

If they are writable, then yes, let's keep them writable, and not create
two function paths where we have to pick and choose.

> 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.

As much fun as that would be, I don't think it would work well.

Although, maybe it would, have an example of how that would look?

I ask as I just went through a large refactoring of the kobject layer to
mark many things const * and I find it a bit "sad" that functions like
this:
	static inline struct device *kobj_to_dev(const struct kobject *kobj)
	{
		return container_of(kobj, struct device, kobj);
	}
have the ability to take a read-only pointer and spit out a writable one
thanks to the pointer math in container_of() with no one being the
wiser.

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

Agreed, but we can do better...

thanks,

greg k-h