linux-kernel - Re: [PATCH v2 1/5] device property: Keep dev_fwnode() and dev_fwnode

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Message-ID: <CAJZ5v0hHPjSN-369pagN3Mnxd1yvc6+4YGb0Kpx3=+aahV=AmQ@mail.gmail.com>
Date:   Mon, 3 Oct 2022 13:54:37 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        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 Fri, Sep 30, 2022 at 4:43 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> 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?
>
> 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.

Well, is this really a problem?

After all, if an immutable structure is embedded in another one, that
doesn't automatically imply that the containing structure has to be
immutable too.  Hence, a const pointer to the inner structure doesn't
automatically yield a const pointer to the outer one.

> > Being able to turn struct device pointers const is certainly not worth
> > violating constness properties.
>
> Agreed, but we can do better...