[PATCH v3 17/24] driver core: Export get_dev_from_fwnode()

[Saravana Kannan]

On Wed, Jul 2, 2025 at 12:26 PM Danilo Krummrich <dakr at kernel.org> wrote:
>
> On Wed, Jul 02, 2025 at 09:34:12AM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Jul 01, 2025 at 01:47:19PM +0200, Ulf Hansson wrote:
> > > It has turned out get_dev_from_fwnode() is useful at a few other places
> > > outside of the driver core, as in gpiolib.c for example. Therefore let's
> > > make it available as a common helper function.
> > >
> > > Suggested-by: Saravana Kannan <saravanak at google.com>
> > > Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> > > Tested-by: Hiago De Franco <hiago.franco at toradex.com> # Colibri iMX8X
> > > Tested-by: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com> # TI AM62A,Xilinx ZynqMP ZCU106
> > > Signed-off-by: Ulf Hansson <ulf.hansson at linaro.org>
> > > ---
> > >  drivers/base/core.c    | 8 ++++++--
> > >  include/linux/device.h | 1 +
> > >  2 files changed, 7 insertions(+), 2 deletions(-)
> > >
> >
> > Acked-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
>
> I'm a bit concerned about exporting get_dev_from_fwnode() -- at least without a
> clear note on that this helper should be used with caution.
>
> AFAIK, a struct fwnode_handle instance does not have a reference count for its
> struct device pointer.
>
> Hence, calling get_dev_from_fwnode() with a valid fwnode handle is not enough.

Not enough for what?

> The caller also needs to ensure that the device the fwnode has a pointer to has
> not been released yet.

Why? The point of the API is to give you the driver core's notion of
the primary device that corresponds to a fwnode at that instant.

There's no refcount needed for that. This is just a simpler way than
looping through all the devices to find the first device that has that
specific fwnode.

It's only the device that needs to be ref counted because the caller
needs to do stuff with it.

-Saravana