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

[Greg Kroah-Hartman]

On Mon, Oct 03, 2022 at 08:08:58PM +0000, Sakari Ailus wrote:
> Hi Greg,
> 
> On Mon, Oct 03, 2022 at 06:17:17PM +0200, Greg Kroah-Hartman wrote:
> > On Mon, Oct 03, 2022 at 05:07:27PM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Oct 03, 2022 at 11:02:19AM +0000, Sakari Ailus wrote:
> > > > 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.
> > > 
> > > I don't like how we loose the ability to do this in an inline C function
> > > by being forced to do it in a macro (as it makes build errors harder to
> > > understand), but I do like the intent here.
> > > 
> > > Let me play around with this a bit on some "smaller" uses of
> > > container_of() and see how that works...
> > 
> > Odd, this doesn't work for me at all.
> > 
> > I tried the following change:
> > 
> > diff --git a/include/linux/device.h b/include/linux/device.h
> > index 424b55df0272..5575c87e6c3b 100644
> > --- a/include/linux/device.h
> > +++ b/include/linux/device.h
> > @@ -680,11 +680,21 @@ struct device_link {
> >  	bool supplier_preactivated; /* Owned by consumer probe. */
> >  };
> >  
> > -static inline struct device *kobj_to_dev(struct kobject *kobj)
> > +static inline struct device *__kobj_to_dev(struct kobject *kobj)
> >  {
> >  	return container_of(kobj, struct device, kobj);
> >  }
> >  
> > +static inline const struct device *__kobj_to_dev_const(const struct kobject *kobj)
> > +{
> > +	return container_of(kobj, const struct device, kobj);
> > +}
> > +
> > +#define kobj_to_dev(kobj)						\
> > +	_Generic((kobj),						\
> > +		 const struct kobject *: __kobj_to_dev_const(kobj),	\
> > +		 struct kobject *: __kobj_to_dev(kobj))
> > +
> >  /**
> >   * device_iommu_mapped - Returns true when the device DMA is translated
> >   *			 by an IOMMU
> > 
> > 
> > which seems all is fine for normal kobject pointers passed in, but for
> > the first 'const struct kobject *' the compiler hits, I get the
> > following error:
> > 
> >   CC      drivers/base/core.o
> > In file included from ./include/linux/acpi.h:15,
> >                  from drivers/base/core.c:11:
> > drivers/base/core.c: In function ‘dev_attr_show’:
> > drivers/base/core.c:2193:48: error: passing argument 1 of ‘__kobj_to_dev’ discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
> >  2193 |         const struct device *dev = kobj_to_dev(kobj);
> >       |                                                ^~~~
> > ./include/linux/device.h:696:50: note: in definition of macro ‘kobj_to_dev’
> >   696 |                  struct kobject *: __kobj_to_dev(kobj))
> >       |                                                  ^~~~
> > ./include/linux/device.h:683:60: note: expected ‘struct kobject *’ but argument is of type ‘const struct kobject *’
> >   683 | static inline struct device *__kobj_to_dev(struct kobject *kobj)
> >       |                                            ~~~~~~~~~~~~~~~~^~~~
> > 
> > 
> > (note, I faked up a constant pointer just to trip the compiler)
> > 
> > The selection of _Generic() seems not to be working here, any hints?  I tried
> > playing around with 'default' and 'typeof' and the like, but all error out the
> > same way.
> 
> Even though only one gets evaluated, it seems the compiler will still
> perform type check on it. I think this problem was partially shared by the
> original patch.
> 
> This should work if written as:
> 
> #define kobj_to_dev(kobj)						\
> 	(_Generic((kobj),						\
> 		  const struct kobject *: __kobj_to_dev_const,		\
> 		  struct kobject *: __kobj_to_dev)(kobj))

Ah, doh!  I had the (kobj) part in the wrong place, thanks for that
fix...

Ok, this looks better, let me see how well the build breaks with some of
these changes

thanks,

greg k-h