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

Sakari Ailus
2022-09-30T14:31:15Z

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.

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