[PATCH v8 3/8] clk: introduce new helper clk_hw_get_children_lcm() to calculate LCM of all child rates

[Brian Masney <bmasney@redhat.com>]

Introduce a new helper that recursively walks through all children and
their descendants, calculating the lowest common multiple (LCM) of their
rates. For the requesting child, it uses the requested rate; for other
enabled children, it uses their current rate. This is useful for
determining what parent rate can satisfy all children through simple
integer dividers.

Link: https://lore.kernel.org/linux-clk/aUSWU7UymULCXOeF@redhat.com/
Link: https://lpc.events/event/19/contributions/2152/
Signed-off-by: Brian Masney <bmasney@redhat.com>
---
 drivers/clk/clk.c            | 54 ++++++++++++++++++++++++++++++++++++++++++++
 include/linux/clk-provider.h |  2 ++
 2 files changed, 56 insertions(+)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 47093cda9df32223c1120c3710261296027c4cd3..bb602c07df9a2b523a72e1e05cc3a27956e7198b 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -14,6 +14,7 @@
 #include <linux/err.h>
 #include <linux/hashtable.h>
 #include <linux/init.h>
+#include <linux/lcm.h>
 #include <linux/list.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
@@ -838,6 +839,59 @@ void clk_hw_set_rate_range(struct clk_hw *hw, unsigned long min_rate,
 }
 EXPORT_SYMBOL_GPL(clk_hw_set_rate_range);
 
+/**
+ * clk_hw_get_children_lcm - Calculate LCM of all children's rates recursively
+ * @hw: The parent clock hardware
+ * @requesting_hw: The child clock that is requesting a rate change (can be NULL)
+ * @requesting_rate: The target rate for the requesting clock
+ *
+ * This helper recursively walks through all children and their descendants,
+ * calculating the lowest common multiple (LCM) of their rates. For the
+ * requesting child, it uses the requested rate; for other enabled children, it
+ * uses their current rate. This is useful for determining what parent rate can
+ * satisfy all children through simple integer dividers.
+ *
+ * Returns: The LCM of all non-zero rates found in the subtree, or 0 if no valid rates.
+ */
+unsigned long clk_hw_get_children_lcm(struct clk_hw *hw, struct clk_hw *requesting_hw,
+				      unsigned long requesting_rate)
+{
+	unsigned long lcm_rate = 0;
+	unsigned long child_rate;
+	struct clk_core *child;
+
+	lockdep_assert_held(&prepare_lock);
+
+	hlist_for_each_entry(child, &hw->core->children, child_node) {
+		/* Use requesting rate for the requesting child, current rate for others */
+		if (child->hw == requesting_hw) {
+			child_rate = requesting_rate;
+		} else {
+			if (!clk_hw_is_enabled(child->hw))
+				continue;
+
+			child_rate = clk_hw_get_rate(child->hw);
+		}
+
+		if (child_rate == 0)
+			continue;
+
+		if (lcm_rate == 0)
+			lcm_rate = child_rate;
+		else
+			lcm_rate = lcm(lcm_rate, child_rate);
+
+		/* Recursively get LCM of this child's children */
+		child_rate = clk_hw_get_children_lcm(child->hw, requesting_hw,
+						     requesting_rate);
+		if (child_rate > 0)
+			lcm_rate = lcm(lcm_rate, child_rate);
+	}
+
+	return lcm_rate;
+}
+EXPORT_SYMBOL_GPL(clk_hw_get_children_lcm);
+
 /*
  * __clk_mux_determine_rate - clk_ops::determine_rate implementation for a mux type clk
  * @hw: mux type clk to determine rate on
diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
index 630705a47129453c241f1b1755f2c2f2a7ed8f77..2699b9759e13d0c1f0b54f4fa4f7f7bdd42e8dde 100644
--- a/include/linux/clk-provider.h
+++ b/include/linux/clk-provider.h
@@ -1430,6 +1430,8 @@ void clk_hw_get_rate_range(struct clk_hw *hw, unsigned long *min_rate,
 			   unsigned long *max_rate);
 void clk_hw_set_rate_range(struct clk_hw *hw, unsigned long min_rate,
 			   unsigned long max_rate);
+unsigned long clk_hw_get_children_lcm(struct clk_hw *hw, struct clk_hw *requesting_hw,
+				      unsigned long requesting_rate);
 
 static inline void __clk_hw_set_clk(struct clk_hw *dst, struct clk_hw *src)
 {

-- 
2.53.0