pmdomain: core: add genpd_for_each_child() helper

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.

Reviewed-by: Abel Vesa <abel.vesa@oss.qualcomm.com>
Signed-off-by: Kevin Hilman (TI) <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulfh@kernel.org>

drivers/pmdomain/core.c

@@ -27,6 +27,7 @@
 #include <linux/debugfs.h>
 
 #include <trace/events/ipi.h>
+#include "core.h"
 
 /* Provides a unique ID for each genpd device */
 static DEFINE_IDA(genpd_ida);
@@ -283,6 +284,49 @@ 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.
+ *
+ * Note: this function is inteded for use by the core and governors,
+ * not for pmdomain providers.
+ *
+ * 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;
 

drivers/pmdomain/core.h

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Internal header for PM domain core
+ *
+ * Copyright (C) 2026 Kevin Hilman <khilman@baylibre.com>, Texas Instruments
+ */
+
+#ifndef __PM_DOMAIN_CORE_H__
+#define __PM_DOMAIN_CORE_H__
+
+struct device;
+struct generic_pm_domain;
+
+int genpd_for_each_child(struct generic_pm_domain *genpd,
+			 int (*fn)(struct device *dev, void *data),
+			 void *data);
+
+#endif /* __PM_DOMAIN_CORE_H__ */