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

[Sakari Ailus]

Hi Greg,

On Fri, Sep 30, 2022 at 04:43:19PM +0200, Greg Kroah-Hartman wrote:
> 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@xxxxxxxxxxxxxxx>
> > > > Fixes: aade55c86033 ("device property: Add const qualifier to device_get_match_data() parameter")
> > > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > > > Acked-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
> > > > Reviewed-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> > > > ---
> > > >  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?

Similar to what container_of() could be, see below.

We could also partially revert aade55c86033bee868a93e4bf3843c9c99e84526
which (also) made dev_fwnode() argument const (which is the source of the
issue).

> 
> 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.

Yeah, container_of() is dangerous, especially in macros. It could of course
be made safer. Something like this:

<URL:https://lore.kernel.org/linux-kernel/1495195570-5249-1-git-send-email-sakari.ailus@xxxxxxxxxxxxxxx/>

I can respin it, back in 2017 I got no replies.

> 
> > Being able to turn struct device pointers const is certainly not worth
> > violating constness properties.
> 
> Agreed, but we can do better...

-- 
Regards,

Sakari Ailus