As demonstrated by the kunit tests, clk_calc_subtree() in the clk core
can overwrite a sibling clk with the parent rate. Clocks that are used
for some subsystems like DRM and sound are particularly sensitive to
this issue.

I consider this to be a logic bug in the clk subsystem, however this
functionality has existed since the clk core was introduced with
commit b2476490ef11 ("clk: introduce the common clock framework"),
and some boards are unknowingly dependent on this behavior.

Let's add support for a v2 rate negotiation logic that addresses the
logic error. Clks can opt into this new behavior by adding the flag
CLK_V2_RATE_NEGOTIATION.

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            | 60 ++++++++++++++++++++++++++++++++++----------
 include/linux/clk-provider.h |  3 +++
 2 files changed, 50 insertions(+), 13 deletions(-)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index bb602c07df9a2b523a72e1e05cc3a27956e7198b..46b3c4c596043abff0be2552fee09c4ce2f15dfb 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -892,6 +892,33 @@ unsigned long clk_hw_get_children_lcm(struct clk_hw *hw, struct clk_hw *requesti
 }
 EXPORT_SYMBOL_GPL(clk_hw_get_children_lcm);
 
+/**
+ * clk_has_v2_rate_negotiation - Check if a clk should use v2 rate negotiation
+ * @core: The clock core to check
+ *
+ * This function recursively checks if the clk or any of its descendants have
+ * the CLK_V2_RATE_NEGOTIATION flag set.
+ *
+ * Returns: true if the v2 logic should be used; false otherwise
+ */
+bool clk_has_v2_rate_negotiation(const struct clk_core *core)
+{
+	struct clk_core *child;
+
+	lockdep_assert_held(&prepare_lock);
+
+	if (core->flags & CLK_V2_RATE_NEGOTIATION)
+		return true;
+
+	hlist_for_each_entry(child, &core->children, child_node) {
+		if (clk_has_v2_rate_negotiation(child))
+			return true;
+	}
+
+	return false;
+}
+EXPORT_SYMBOL_GPL(clk_has_v2_rate_negotiation);
+
 /*
  * __clk_mux_determine_rate - clk_ops::determine_rate implementation for a mux type clk
  * @hw: mux type clk to determine rate on
@@ -2299,7 +2326,8 @@ static int __clk_speculate_rates(struct clk_core *core,
 }
 
 static void clk_calc_subtree(struct clk_core *core, unsigned long new_rate,
-			     struct clk_core *new_parent, u8 p_index)
+			     struct clk_core *new_parent, u8 p_index,
+			     struct clk_core *initiating_clk)
 {
 	struct clk_core *child;
 
@@ -2312,8 +2340,12 @@ static void clk_calc_subtree(struct clk_core *core, unsigned long new_rate,
 		new_parent->new_child = core;
 
 	hlist_for_each_entry(child, &core->children, child_node) {
-		child->new_rate = clk_recalc(child, new_rate);
-		clk_calc_subtree(child, child->new_rate, NULL, 0);
+		if (child != initiating_clk && clk_has_v2_rate_negotiation(child))
+			child->new_rate = child->rate;
+		else
+			child->new_rate = clk_recalc(child, new_rate);
+
+		clk_calc_subtree(child, child->new_rate, NULL, 0, initiating_clk);
 	}
 }
 
@@ -2322,7 +2354,8 @@ static void clk_calc_subtree(struct clk_core *core, unsigned long new_rate,
  * changed.
  */
 static struct clk_core *clk_calc_new_rates(struct clk_core *core,
-					   unsigned long rate)
+					   unsigned long rate,
+					   struct clk_core *initiating_clk)
 {
 	struct clk_core *top = core;
 	struct clk_core *old_parent, *parent;
@@ -2370,7 +2403,7 @@ static struct clk_core *clk_calc_new_rates(struct clk_core *core,
 		return NULL;
 	} else {
 		/* pass-through clock with adjustable parent */
-		top = clk_calc_new_rates(parent, rate);
+		top = clk_calc_new_rates(parent, rate, initiating_clk);
 		new_rate = parent->new_rate;
 		goto out;
 	}
@@ -2395,10 +2428,10 @@ static struct clk_core *clk_calc_new_rates(struct clk_core *core,
 
 	if ((core->flags & CLK_SET_RATE_PARENT) && parent &&
 	    best_parent_rate != parent->rate)
-		top = clk_calc_new_rates(parent, best_parent_rate);
+		top = clk_calc_new_rates(parent, best_parent_rate, initiating_clk);
 
 out:
-	clk_calc_subtree(core, new_rate, parent, p_index);
+	clk_calc_subtree(core, new_rate, parent, p_index, initiating_clk);
 
 	return top;
 }
@@ -2446,7 +2479,7 @@ static struct clk_core *clk_propagate_rate_change(struct clk_core *core,
  * walk down a subtree and set the new rates notifying the rate
  * change on the way
  */
-static void clk_change_rate(struct clk_core *core)
+static void clk_change_rate(struct clk_core *core, struct clk_core *initiating_clk)
 {
 	struct clk_core *child;
 	struct hlist_node *tmp;
@@ -2515,7 +2548,7 @@ static void clk_change_rate(struct clk_core *core)
 		__clk_notify(core, POST_RATE_CHANGE, old_rate, core->rate);
 
 	if (core->flags & CLK_RECALC_NEW_RATES)
-		(void)clk_calc_new_rates(core, core->new_rate);
+		(void)clk_calc_new_rates(core, core->new_rate, initiating_clk);
 
 	/*
 	 * Use safe iteration, as change_rate can actually swap parents
@@ -2525,12 +2558,12 @@ static void clk_change_rate(struct clk_core *core)
 		/* Skip children who will be reparented to another clock */
 		if (child->new_parent && child->new_parent != core)
 			continue;
-		clk_change_rate(child);
+		clk_change_rate(child, initiating_clk);
 	}
 
 	/* handle the new child who might not be in core->children yet */
 	if (core->new_child)
-		clk_change_rate(core->new_child);
+		clk_change_rate(core->new_child, initiating_clk);
 
 	clk_pm_runtime_put(core);
 }
@@ -2586,7 +2619,7 @@ static int clk_core_set_rate_nolock(struct clk_core *core,
 		return -EBUSY;
 
 	/* calculate new rates and get the topmost changed clock */
-	top = clk_calc_new_rates(core, req_rate);
+	top = clk_calc_new_rates(core, req_rate, core);
 	if (!top)
 		return -EINVAL;
 
@@ -2605,7 +2638,7 @@ static int clk_core_set_rate_nolock(struct clk_core *core,
 	}
 
 	/* change the rates */
-	clk_change_rate(top);
+	clk_change_rate(top, core);
 
 	core->req_rate = req_rate;
 err:
@@ -3592,6 +3625,7 @@ static const struct {
 	ENTRY(CLK_IS_CRITICAL),
 	ENTRY(CLK_OPS_PARENT_ENABLE),
 	ENTRY(CLK_DUTY_CYCLE_PARENT),
+	ENTRY(CLK_V2_RATE_NEGOTIATION),
 #undef ENTRY
 };
 
diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
index 2699b9759e13d0c1f0b54f4fa4f7f7bdd42e8dde..e0fc0bd347e5920e999ac96dbed9fc247f9443fa 100644
--- a/include/linux/clk-provider.h
+++ b/include/linux/clk-provider.h
@@ -32,6 +32,8 @@
 #define CLK_OPS_PARENT_ENABLE	BIT(12)
 /* duty cycle call may be forwarded to the parent clock */
 #define CLK_DUTY_CYCLE_PARENT	BIT(13)
+/* clock participates in v2 rate negotiation */
+#define CLK_V2_RATE_NEGOTIATION	BIT(14)
 
 struct clk;
 struct clk_hw;
@@ -1432,6 +1434,7 @@ 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);
+bool clk_has_v2_rate_negotiation(const struct clk_core *core);
 
 static inline void __clk_hw_set_clk(struct clk_hw *dst, struct clk_hw *src)
 {

-- 
2.53.0