clk: lpc18xx-ccu: fix potential system hang when disabling unused clocks

CCU branch clock register must only be accessed while the base
(parent) clock is running. Access with a disabled base clock
will cause the system to hang. Fix this issue by adding code
that check if the parent clock is running in the is_enabled
clk_ops callback.

This hang would occur when disabling unused clocks after AMBA
runtime pm had already disabled some of the clocks.
Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/nxp/clk-lpc18xx-ccu.c

@@ -179,9 +179,22 @@ static void lpc18xx_ccu_gate_disable(struct clk_hw *hw)
 
 static int lpc18xx_ccu_gate_is_enabled(struct clk_hw *hw)
 {
-	struct clk_gate *gate = to_clk_gate(hw);
+	const struct clk_hw *parent;
+
+	/*
+	 * The branch clock registers are only accessible
+	 * if the base (parent) clock is enabled. Register
+	 * access with a disabled base clock will hang the
+	 * system.
+	 */
+	parent = clk_hw_get_parent(hw);
+	if (!parent)
+		return 0;
+
+	if (!clk_hw_is_enabled(parent))
+		return 0;
 
-	return clk_readl(gate->reg) & LPC18XX_CCU_RUN;
+	return clk_gate_ops.is_enabled(hw);
 }
 
 static const struct clk_ops lpc18xx_ccu_gate_ops = {