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

[Shivendra Pratap]

On 18-05-2026 14:27, Bartosz Golaszewski wrote:
> On Thu, 14 May 2026 16:25:49 +0200, Shivendra Pratap
> <shivendra.pratap@xxxxxxxxxxxxxxxx> said:
> > 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 <bartosz.golaszewski@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Shivendra Pratap <shivendra.pratap@xxxxxxxxxxxxxxxx>
> > ---
> >   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);
> > +}
>
> Ah, this seems to answer my previous question, but...
>
> > +
> >   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.
>
> Which is a separate problem we're discussing elsewhere. It should probably drop
> the fwnode reference it holds, not the one of of_node.
>
> > +			 * 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);
>
> What if the device never gets bound to the driver? The release will never be
> called, this is why it's wrong to schedule devres actions for unbound devices
> and one of the reasons for patch 1 in this series.
>
> What I suggest for now is: in tear-down path: see if the cell has the
> get_child_fwnode() callback and - if so - drop the reference. Add a big, fat
> comment saying that this must be removed if we decide to switch to dropping the
> device's fwnode reference in platform driver core which may happen soon.

Ack. sure. lets me work it out.

thanks,
Shivendra