Add a new internal helper function genpd_for_each_child() that recursively
iterates over all devices in a PM domain and its child domains (subdomains).
This helper is useful for governors and other core PM domain code that needs
to examine or apply operations to all devices within a domain hierarchy.

The function takes a callback that is invoked for each device, and supports
early termination if the callback returns a non-zero value.

The helper is defined in a new internal header drivers/pmdomain/core.h and
implemented in drivers/pmdomain/core.c, making it available to other PM
domain subsystem components.

The first user of this helper is the cpu_system_power_down_ok() governor
function, which uses it to check device QoS latency constraints across the
entire domain hierarchy.

Signed-off-by: Kevin Hilman (TI) <khilman@baylibre.com>
---
 drivers/pmdomain/core.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 drivers/pmdomain/core.h | 17 +++++++++++++++++
 2 files changed, 59 insertions(+)

diff --git a/drivers/pmdomain/core.c b/drivers/pmdomain/core.c
index bf82775f6a67..41c987fc5c5a 100644
--- a/drivers/pmdomain/core.c
+++ b/drivers/pmdomain/core.c
@@ -24,6 +24,8 @@
 #include <linux/cpu.h>
 #include <linux/debugfs.h>
 
+#include "core.h"
+
 /* Provides a unique ID for each genpd device */
 static DEFINE_IDA(genpd_ida);
 
@@ -281,6 +283,46 @@ static void genpd_sd_counter_inc(struct generic_pm_domain *genpd)
 	smp_mb__after_atomic();
 }
 
+/**
+ * genpd_for_each_child - Recursively iterate over all devices
+ *                        in a PM domain and its subdomains.
+ * @genpd: PM domain to iterate over.
+ * @fn: Callback function to invoke for each device.
+ * @data: Data to pass to the callback function.
+ *
+ * This function recursively walks through all devices in the given PM domain
+ * and all devices in its child PM domains (subdomains). For each device found,
+ * the callback function @fn is invoked with the device and @data as arguments.
+ *
+ * Returns: 0 on success, or the first non-zero value returned by @fn.
+ */
+int genpd_for_each_child(struct generic_pm_domain *genpd,
+			 int (*fn)(struct device *dev, void *data),
+			 void *data)
+{
+	struct pm_domain_data *pdd;
+	struct gpd_link *link;
+	int ret;
+
+	/* First, iterate over all devices in this domain */
+	list_for_each_entry(pdd, &genpd->dev_list, list_node) {
+		ret = fn(pdd->dev, data);
+		if (ret)
+			return ret;
+	}
+
+	/* Then, recursively iterate over all child domains (subdomains) */
+	list_for_each_entry(link, &genpd->parent_links, parent_node) {
+		struct generic_pm_domain *child_pd = link->child;
+
+		ret = genpd_for_each_child(child_pd, fn, data);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
 #ifdef CONFIG_DEBUG_FS
 static struct dentry *genpd_debugfs_dir;
 
diff --git a/drivers/pmdomain/core.h b/drivers/pmdomain/core.h
new file mode 100644
index 000000000000..7061891d31fb
--- /dev/null
+++ b/drivers/pmdomain/core.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Internal header for PM domain core
+ *
+ * Copyright (C) 2011 Rafael J. Wysocki <rjw@sisk.pl>, Renesas Electronics Corp.
+ */
+
+#ifndef __PM_DOMAIN_CORE_H__
+#define __PM_DOMAIN_CORE_H__
+
+#include <linux/pm_domain.h>
+
+int genpd_for_each_child(struct generic_pm_domain *genpd,
+			 int (*fn)(struct device *dev, void *data),
+			 void *data);
+
+#endif /* __PM_DOMAIN_CORE_H__ */

-- 
2.51.0