mfd: syscon: Add of_syscon_register_regmap() API

The of_syscon_register_regmap() API allows an externally created regmap
to be registered with syscon. This regmap can then be returned to client
drivers using the syscon_regmap_lookup_by_phandle() APIs.

The API is used by platforms where mmio access to the syscon registers is
not possible, and a underlying soc driver like exynos-pmu provides a SoC
specific regmap that can issue a SMC or hypervisor call to write the
register.

This approach keeps the SoC complexities out of syscon, but allows common
drivers such as syscon-poweroff, syscon-reboot and friends that are used
by many SoCs already to be re-used.

Signed-off-by: Peter Griffin <[e-mail]>
Reviewed-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Sam Protsenko <[e-mail]>
Tested-by: Will McVicker <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/syscon.c

···
 	return syscon->regmap;
 }
 
+/**
+ * of_syscon_register_regmap() - Register regmap for specified device node
+ * @np: Device tree node
+ * @regmap: Pointer to regmap object
+ *
+ * Register an externally created regmap object with syscon for the specified
+ * device tree node. This regmap will then be returned to client drivers using
+ * the syscon_regmap_lookup_by_phandle() API.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int of_syscon_register_regmap(struct device_node *np, struct regmap *regmap)
+{
+	struct syscon *entry, *syscon = NULL;
+	int ret;
+
+	if (!np || !regmap)
+		return -EINVAL;
+
+	syscon = kzalloc(sizeof(*syscon), GFP_KERNEL);
+	if (!syscon)
+		return -ENOMEM;
+
+	/* check if syscon entry already exists */
+	spin_lock(&syscon_list_slock);
+
+	list_for_each_entry(entry, &syscon_list, list)
+		if (entry->np == np) {
+			ret = -EEXIST;
+			goto err_unlock;
+		}
+
+	syscon->regmap = regmap;
+	syscon->np = np;
+
+	/* register the regmap in syscon list */
+	list_add_tail(&syscon->list, &syscon_list);
+	spin_unlock(&syscon_list_slock);
+
+	return 0;
+
+err_unlock:
+	spin_unlock(&syscon_list_slock);
+	kfree(syscon);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(of_syscon_register_regmap);
+
 struct regmap *device_node_to_regmap(struct device_node *np)
 {
 	return device_node_get_regmap(np, false);

include/linux/mfd/syscon.h

···
 						    unsigned int *out_args);
 struct regmap *syscon_regmap_lookup_by_phandle_optional(struct device_node *np,
 							const char *property);
+int of_syscon_register_regmap(struct device_node *np,
+			      struct regmap *regmap);
 #else
 static inline struct regmap *device_node_to_regmap(struct device_node *np)
 {
···
 					const char *property)
 {
 	return NULL;
+}
+
+static inline int of_syscon_register_regmap(struct device_node *np,
+					struct regmap *regmap)
+{
+	return -EOPNOTSUPP;
 }
 
 #endif