[PATCH v3 1/2] mfd: syscon: add of_syscon_register_regmap() API

[Peter Griffin <peter.griffin@linaro.org>]

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 <peter.griffin@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
---
Changes in v3:
- Move kzalloc allocation outside of spin lock (Arnd)
- Link to v2 https://lore.kernel.org/linux-arm-kernel/20240620112446.1286223-2-peter.griffin@linaro.org/

Changes in v2:
- Keep syscon lock held between checking and adding entry (Krzysztof)
- Link to v1 https://lore.kernel.org/linux-arm-kernel/20240614140421.3172674-2-peter.griffin@linaro.org/
---
 drivers/mfd/syscon.c       | 48 ++++++++++++++++++++++++++++++++++++++
 include/linux/mfd/syscon.h |  8 +++++++
 2 files changed, 56 insertions(+)

diff --git a/drivers/mfd/syscon.c b/drivers/mfd/syscon.c
index 7d0e91164cba..33f1e07ab24d 100644
--- a/drivers/mfd/syscon.c
+++ b/drivers/mfd/syscon.c
@@ -192,6 +192,54 @@ static struct regmap *device_node_get_regmap(struct device_node *np,
 	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);
diff --git a/include/linux/mfd/syscon.h b/include/linux/mfd/syscon.h
index c315903f6dab..aad9c6b50463 100644
--- a/include/linux/mfd/syscon.h
+++ b/include/linux/mfd/syscon.h
@@ -28,6 +28,8 @@ struct regmap *syscon_regmap_lookup_by_phandle_args(struct device_node *np,
 						    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)
 {
@@ -67,6 +69,12 @@ static inline struct regmap *syscon_regmap_lookup_by_phandle_optional(
 	return NULL;
 }
 
+static inline int of_syscon_register_regmap(struct device_node *np,
+					struct regmap *regmap)
+{
+	return -EOPNOTSUPP;
+}
+
 #endif
 
 #endif /* __LINUX_MFD_SYSCON_H__ */
-- 
2.45.2.741.gdbec12cfda-goog