irqchip/mbigen: Don't use bus_get_dev_root() to find the parent

bus_get_dev_root() returns sp->dev_root which is set in subsys_register(),
but subsys_register() is not called by platform_bus_init().

Therefor for the platform_bus_type, bus_get_dev_root() always returns NULL.
This makes mbigen_of_create_domain() always return -ENODEV.

Don't try to retrieve the parent via bus_get_dev_root() and
unconditionally hand a NULL pointer to of_platform_device_create() to
fix this.

Fixes: fea087f ("irqchip/mbigen: move to use bus_get_dev_root()")
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240220111429.110666-1-chenjun102@huawei.com

Author: Chen Jun

drivers/irqchip/irq-mbigen.c

@@ -235,22 +235,17 @@ static const struct irq_domain_ops mbigen_domain_ops = {
 static int mbigen_of_create_domain(struct platform_device *pdev,
 				   struct mbigen_device *mgn_chip)
 {
-	struct device *parent;
 	struct platform_device *child;
 	struct irq_domain *domain;
 	struct device_node *np;
 	u32 num_pins;
 	int ret = 0;
 
-	parent = bus_get_dev_root(&platform_bus_type);
-	if (!parent)
-		return -ENODEV;
-
 	for_each_child_of_node(pdev->dev.of_node, np) {
 		if (!of_property_read_bool(np, "interrupt-controller"))
 			continue;
 
-		child = of_platform_device_create(np, NULL, parent);
+		child = of_platform_device_create(np, NULL, NULL);
 		if (!child) {
 			ret = -ENOMEM;
 			break;
@@ -273,7 +268,6 @@ static int mbigen_of_create_domain(struct platform_device *pdev,
 		}
 	}
 
-	put_device(parent);
 	if (ret)
 		of_node_put(np);