serial: 8250_omap: Fix idling for unloaded serdev drivers

For many years omap variants have been setting the runtime PM
autosuspend delay to -1 to prevent unsafe policy with lossy first
character on wake-up. The user must specifically enable the timeout
for UARTs if desired.

We must not enable the workaround for serdev devices though. It leads
into UARTs not idling if no serdev devices are loaded and there is no
sysfs entry to configure the UART in that case. And this means that
my PM may not work unless the serdev modules are loaded.

We can detect a serdev device being configured based on a dts child
node, and we can simply skip the workround in that case. And the
serdev driver can idle the port during runtime when suitable if an
out-of-band wake-up GPIO line exists for example.

Let's also add some comments to the workaround while at it.

Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20190723115400.46432-1-tony@atomide.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_omap.c

@@ -1234,7 +1234,16 @@ static int omap8250_probe(struct platform_device *pdev)
 
 	device_init_wakeup(&pdev->dev, true);
 	pm_runtime_use_autosuspend(&pdev->dev);
-	pm_runtime_set_autosuspend_delay(&pdev->dev, -1);
+
+	/*
+	 * Disable runtime PM until autosuspend delay unless specifically
+	 * enabled by the user via sysfs. This is the historic way to
+	 * prevent an unsafe default policy with lossy characters on wake-up.
+	 * For serdev devices this is not needed, the policy can be managed by
+	 * the serdev driver.
+	 */
+	if (!of_get_available_child_count(pdev->dev.of_node))
+		pm_runtime_set_autosuspend_delay(&pdev->dev, -1);
 
 	pm_runtime_irq_safe(&pdev->dev);
 	pm_runtime_enable(&pdev->dev);