cpuidle: psci: Avoid initializing faux device if no DT idle states are present

Commit af5376a ("cpuidle: psci: Transition to the faux device interface")
transitioned the PSCI cpuidle driver from using a platform device to the
faux device framework. However, unlike platform devices, the faux device
infrastructure logs an error when the probe function fails, even if the
failure is intentional or expected.

To prevent unnecessary error logs, we can skip creating the faux device
entirely if there are no PSCI idle states defined in the device tree.
Introduce a check for DT idle states during initialization and avoid
setting up the device if none are found.

This ensures cleaner logs and avoids misleading probe failure messages
when PSCI idle support is intentionally not described in DT.

Fixes: af5376a ("cpuidle: psci: Transition to the faux device interface")
Reported-by: Jon Hunter <jonathanh@nvidia.com>
Closes: https://lore.kernel.org/r/cf4e70e4-9fe5-4697-8744-8c12c41b5ff9@nvidia.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/20250502140119.2578909-1-sudeep.holla@arm.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: sudeep-holla

drivers/cpuidle/cpuidle-psci.c

@@ -438,10 +438,29 @@ static struct faux_device_ops psci_cpuidle_ops = {
 	.probe = psci_cpuidle_probe,
 };
 
+static bool __init dt_idle_state_present(void)
+{
+	struct device_node *cpu_node __free(device_node);
+	struct device_node *state_node __free(device_node);
+
+	cpu_node = of_cpu_device_node_get(cpumask_first(cpu_possible_mask));
+	if (!cpu_node)
+		return false;
+
+	state_node = of_get_cpu_state_node(cpu_node, 0);
+	if (!state_node)
+		return false;
+
+	return !!of_match_node(psci_idle_state_match, state_node);
+}
+
 static int __init psci_idle_init(void)
 {
 	struct faux_device *fdev;
 
+	if (!dt_idle_state_present())
+		return 0;
+
 	fdev = faux_device_create("psci-cpuidle", NULL, &psci_cpuidle_ops);
 	if (!fdev) {
 		pr_err("Failed to create psci-cpuidle device\n");