Messages in this thread | | | Date | Mon, 17 Oct 2022 07:54:52 +0000 | From | Sakari Ailus <> | Subject | Re: [PATCH v2] driver core: allow kobj_to_dev() to take a const pointer |
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Hi Greg,
On Sun, Oct 16, 2022 at 12:41:26PM +0200, Greg Kroah-Hartman wrote: > If a const * to a kobject is passed to kobj_to_dev(), we want to return > back a const * to a device as the driver core shouldn't be modifying a > constant structure. But when dealing with container_of() the pointer > const attribute is cast away, so we need to manually handle this by > determining the type of the pointer passed in to know the type of the > pointer to pass out.
Alternatively container_of() could be fixed, but that will likely produce lots of warnings currently.
> > Luckily _Generic can do this type of magic, and as the kernel now > supports C11 it is availble to us to handle this type of build-time type > detection. > > Cc: Sakari Ailus <sakari.ailus@linux.intel.com> > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> > Cc: "Rafael J. Wysocki" <rafael@kernel.org> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > --- > v2 - use _Generic() to make this type safe as pointed out by Sakari > > include/linux/device.h | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/include/linux/device.h b/include/linux/device.h > index 424b55df0272..023ea50b1916 100644 > --- a/include/linux/device.h > +++ b/include/linux/device.h > @@ -680,11 +680,27 @@ 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); > +} > + > +/* > + * container_of() will happily take a const * and spit back a non-const * as it > + * is just doing pointer math. But we want to be a bit more careful in the > + * driver code, so manually force any const * of a kobject to also be a const * > + * to a device. > + */
container_of() documentation has (probably?) never warned about this.
Wouldn't such a comment be more appropriate there? Albeit it wouldn't be needed if container_of() were fixed.
> +#define kobj_to_dev(kobj) \ > + _Generic((kobj), \ > + const struct kobject *: __kobj_to_dev_const, \ > + struct kobject *: __kobj_to_dev)(kobj) > + > /** > * device_iommu_mapped - Returns true when the device DMA is translated > * by an IOMMU
-- Kind regards,
Sakari Ailus
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