Subject: Re: [PATCH v22 08/13] mfd: core: Add firmware-node support to MFD cells

From: Lee Jones <[email-masked]>
Date: Thu, 21 May 2026 14:24:19 +0100

On Thu, 21 May 2026, Bartosz Golaszewski wrote:

> On Thu, May 21, 2026 at 1:26 PM Lee Jones <[email-masked]> wrote:
> >
> > On Thu, 14 May 2026, Shivendra Pratap wrote:
> >
> > > MFD core has no way to register a child device using an explicit firmware
> > > node. This prevents drivers from registering child nodes when those nodes
> > > do not define a compatible string. One such example is the PSCI
> > > "reboot-mode" node, which omits a compatible string as it describes
> > > boot-states provided by the underlying firmware.
> > >
> > > Extend struct mfd_cell with a callback that allows drivers to provide an
> > > explicit firmware node. The node is added to the MFD child device during
> > > registration when none is assigned by device tree, ACPI, or software
> > > matching.
> > >
> > > Suggested-by: Bartosz Golaszewski <[email-masked]>
> > > Signed-off-by: Shivendra Pratap <[email-masked]>
> > > ---
> > >  drivers/mfd/mfd-core.c   | 30 ++++++++++++++++++++++++++++++
> > >  include/linux/mfd/core.h | 14 ++++++++++++++
> > >  2 files changed, 44 insertions(+)
> > >
> > > diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
> > > index 7aa32b90cf1eb7fa0a05bf3dc506e60a262c9850..cc2a2a924d6d3044e29a9f864b536ee325ed797b 100644
> > > --- a/drivers/mfd/mfd-core.c
> > > +++ b/drivers/mfd/mfd-core.c
> > > @@ -10,6 +10,7 @@
> > >  #include <linux/kernel.h>
> > >  #include <linux/platform_device.h>
> > >  #include <linux/acpi.h>
> > > +#include <linux/fwnode.h>
> > >  #include <linux/list.h>
> > >  #include <linux/property.h>
> > >  #include <linux/mfd/core.h>
> > > @@ -148,6 +149,11 @@ static int mfd_match_of_node_to_dev(struct platform_device *pdev,
> > >       return 0;
> > >  }
> > >
> > > +static void mfd_child_fwnode_put(void *data)
> > > +{
> > > +     fwnode_handle_put(data);
> > > +}
> > > +
> > >  static int mfd_add_device(struct device *parent, int id,
> > >                         const struct mfd_cell *cell,
> > >                         struct resource *mem_base,
> > > @@ -156,6 +162,7 @@ static int mfd_add_device(struct device *parent, int id,
> > >       struct resource *res;
> > >       struct platform_device *pdev;
> > >       struct mfd_of_node_entry *of_entry, *tmp;
> > > +     struct fwnode_handle *fwnode;
> > >       bool disabled = false;
> > >       int ret = -ENOMEM;
> > >       int platform_id;
> > > @@ -224,6 +231,29 @@ static int mfd_add_device(struct device *parent, int id,
> > >
> > >       mfd_acpi_add_device(cell, pdev);
> > >
> > > +     if (!pdev->dev.fwnode && cell->get_child_fwnode) {
> > > +             fwnode = cell->get_child_fwnode(parent);
> > > +             if (fwnode) {
> > > +                     device_set_node(&pdev->dev, fwnode);
> > > +
> > > +                     /*
> > > +                      * platform_device_release() drops only of_node refs.
> > > +                      * Track non-OF fwnodes explicitly so they are put on
> > > +                      * all teardown paths.
> > > +                      */
> > > +                     if (!to_of_node(fwnode)) {
> > > +                             ret = devm_add_action(&pdev->dev,
> > > +                                                   mfd_child_fwnode_put,
> > > +                                                   fwnode);
> > > +                             if (ret) {
> > > +                                     device_set_node(&pdev->dev, NULL);
> > > +                                     fwnode_handle_put(fwnode);
> > > +                                     goto fail_of_entry;
> > > +                             }
> > > +                     }
> > > +             }
> > > +     }
> >
> > mfd_add_device() is getting very busy now with support for all of these
> > different registration APIs.  Suggest that we start breaking them out.
> >
> > > +
> > >       if (cell->pdata_size) {
> > >               ret = platform_device_add_data(pdev,
> > >                                       cell->platform_data, cell->pdata_size);
> > > diff --git a/include/linux/mfd/core.h b/include/linux/mfd/core.h
> > > index faeea7abd688f223fb0b31cde0a9b69dfe2a61ff..abfc26c057d6ee46947ba2b6f2e99f420e74b127 100644
> > > --- a/include/linux/mfd/core.h
> > > +++ b/include/linux/mfd/core.h
> > > @@ -50,6 +50,7 @@
> > >  #define MFD_DEP_LEVEL_HIGH 1
> > >
> > >  struct irq_domain;
> > > +struct fwnode_handle;
> > >  struct software_node;
> > >
> > >  /* Matches ACPI PNP id, either _HID or _CID, or ACPI _ADR */
> > > @@ -80,6 +81,19 @@ struct mfd_cell {
> > >
> > >       /* Software node for the device. */
> > >       const struct software_node *swnode;
> > > +     /*
> > > +      * Callback to return an explicit firmware node.
> > > +      * @parent: MFD parent device passed to mfd_add_devices().
> > > +      *
> > > +      * Called only if OF/ACPI matching did not assign a fwnode.
> > > +      * Ownership of the returned reference is transferred to MFD core.
> > > +      *
> > > +      * Return a referenced fwnode or NULL if none is available.
> > > +      *
> > > +      * mfd_cell must be zero-initialized or get_child_fwnode must be NULL
> > > +      * when unused.
> > > +      */
> > > +     struct fwnode_handle *(*get_child_fwnode)(struct device *parent);
> >
> > I'm very much against pointers to functions if they can be avoided.  Why
> > does fwnode need this and none of the other APIs do?
> >
> 
> I suggested it because of its flexibility. The alternative I had in
> mind is something like a new field in mfd_cell:
> 
>     const char *cell_node_name;
> 
> Which - if set - would tell MFD to look up an fwnode that's a child of
> the parent device's node by name - as it may not have a compatible.

Remind me why the chlid device can't look-up its own fwnode?

-- 
Lee Jones