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Message-ID: <YzsLDUhjDCCVRy2G@kroah.com> Date: Mon, 3 Oct 2022 18:17:17 +0200 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: Sakari Ailus <sakari.ailus@...ux.intel.com> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Heikki Krogerus <heikki.krogerus@...ux.intel.com>, Bjorn Andersson <andersson@...nel.org>, Prashant Malani <pmalani@...omium.org>, linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org, Daniel Scally <djrscally@...il.com>, "Rafael J. Wysocki" <rafael@...nel.org> Subject: Re: [PATCH v2 1/5] device property: Keep dev_fwnode() and dev_fwnode_const() separate 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@...ux.intel.com> > > > > > > Fixes: aade55c86033 ("device property: Add const qualifier to device_get_match_data() parameter") > > > > > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com> > > > > > > Acked-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com> > > > > > > Reviewed-by: Sakari Ailus <sakari.ailus@...ux.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? > > > > 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@linux.intel.com/> > > > > 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. thanks, greg k-h
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