nvmem: core: allow specifying of_node

Until now, the of_node of the parent device is used. Some devices
provide more than just the nvmem provider. To avoid name space clashes,
add a way to allow specifying the nvmem cells in subnodes. Consider the
following example:

    flash@0 {
        compatible = "jedec,spi-nor";

        partitions {
            compatible = "fixed-partitions";
            #address-cells = <1>;
            #size-cells = <1>;

            partition@0 {
                reg = <0x000000 0x010000>;
            };
        };

        otp {
            compatible = "user-otp";
            #address-cells = <1>;
            #size-cells = <1>;

            serial-number@0 {
                reg = <0x0 0x8>;
            };
        };
    };

There the nvmem provider might be the MTD partition or the OTP region of
the flash.

Add a new config->of_node parameter, which if set, will be used instead
of the parent's of_node.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210424110608.15748-2-michael@walle.cc

drivers/nvmem/core.c

@@ -789,7 +789,9 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
 	nvmem->reg_write = config->reg_write;
 	nvmem->keepout = config->keepout;
 	nvmem->nkeepout = config->nkeepout;
-	if (!config->no_of_node)
+	if (config->of_node)
+		nvmem->dev.of_node = config->of_node;
+	else if (!config->no_of_node)
 		nvmem->dev.of_node = config->dev->of_node;
 
 	switch (config->id) {

include/linux/nvmem-provider.h

@@ -57,6 +57,7 @@ struct nvmem_keepout {
  * @type:	Type of the nvmem storage
  * @read_only:	Device is read-only.
  * @root_only:	Device is accessibly to root only.
+ * @of_node:	If given, this will be used instead of the parent's of_node.
  * @no_of_node:	Device should not use the parent's of_node even if it's !NULL.
  * @reg_read:	Callback to read data.
  * @reg_write:	Callback to write data.
@@ -86,6 +87,7 @@ struct nvmem_config {
 	enum nvmem_type		type;
 	bool			read_only;
 	bool			root_only;
+	struct device_node	*of_node;
 	bool			no_of_node;
 	nvmem_reg_read_t	reg_read;
 	nvmem_reg_write_t	reg_write;