On Wed, Nov 23, 2022 at 04:48:41PM +0100, Maximilian Luz wrote:

On 11/23/22 16:37, Greg Kroah-Hartman wrote:

quoted

On Wed, Nov 23, 2022 at 02:52:59PM +0000, Matthew Wilcox wrote:

quoted

On Wed, Nov 23, 2022 at 02:59:00PM +0100, Maximilian Luz wrote:

quoted

On 11/23/22 14:34, Andy Shevchenko wrote:

quoted

On Wed, Nov 23, 2022 at 02:14:31PM +0100, Maximilian Luz wrote:

quoted

On 11/23/22 13:25, Greg Kroah-Hartman wrote:

quoted

The uevent() callback in struct device_type should not be modifying the
device that is passed into it, so mark it as a const * and propagate the
function signature changes out into all relevant subsystems that use
this callback.

[...]

quoted

quoted

-static inline struct ssam_device *to_ssam_device(struct device *d)
+static inline struct ssam_device *to_ssam_device(const struct device *d)
    {
    	return container_of(d, struct ssam_device, dev);
    }

I am slightly conflicted about this change as that now more or less
implicitly drops the const. So I'm wondering if it wouldn't be better to
either create a function specifically for const pointers or to just
open-code it in the instance above.

I guess we could also convert this to a macro. Then at least there
wouldn't be an explicit and potentially misleading const-conversion
indicated in the function signature.

This is an intermediate step as far as I know since moving container_of to
recognize const is a bit noisy right now. I guess you can find a discussion
on the topic between Greg and Sakari.

Thanks! I assume you are referring to the following?

	https://lore.kernel.org/lkml/4218173bd72b4f1899d4c41a8e251f0d@AcuMS.aculab.com/T/ (local)

As far as I can tell this is only a warning in documentation, not
compile time (which would probably be impossible?).

As I've said I'd be fine with converting the function to a macro (and
preferably adding a similar warning like the one proposed in that
thread). The point that irks me up is just that, as proposed, the
function signature would now advertise a conversion that should never be
happening.

Having two separate functions would create a compile-time guarantee, so
I'd prefer that, but I can understand if that might be considered too
noisy in code. Or if there is a push to make container_of() emit a
compile-time warning I'd also be perfectly happy with converting it to a
macro now as that'd alleviate the need for functions in the future.

Can't we do:

static inline const struct ssam_device *to_ssam_device(const struct device *d)
{
	return container_of(d, const struct ssam_device, dev);
}

You could, if you can always handle a const pointer coming out of this
function, but I don't think you can.

What you might want to do instead, and I'll be glad to do it for all of
the functions like this I change, is to do what we have for struct
device now:

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.
  */
#define kobj_to_dev(kobj)                                       \
         _Generic((kobj),                                        \
                  const struct kobject *: __kobj_to_dev_const,   \
                  struct kobject *: __kobj_to_dev)(kobj)


Want me to do the same thing here as well?

That looks great! Thanks!

I would very much prefer that.

Ok, will respin this patch as at least 2 individual patches, one that
does the change to to_ssam_device() and the next that does the bus-wide
changes.

I'll review the other container_of() users in this patch as well to see
if they can be converted as well.

thanks,

greg k-h