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 12:26:38 +0100

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?

>  	/*
>  	 * Device Tree compatible string
> 
> -- 
> 2.34.1
> 

-- 
Lee Jones