uart: mediatek: support Rx in-band wakeup

In order to support Rx in-band wakeup, we need to enable irq wake on an
edge sensitive interrupt of Rx pin before suspend and disable it when
resuming.

This interrupt is used only as wake source to resume the system when
suspended. Note that the sent character will be lost as the controller is
actually suspended.

We use this to support wakeup on bluetooth. Bluetooth will repeatedly send
0xFD to wakeup host. Once host detects Rx falling, an interrupt is
triggered, and the system leaves sleep state. Then, the bluetooth driver
will send 0xFC to bluetooth and bluetooth can start to send normal HCI
packets.

Signed-off-by: Claire Chang <[e-mail]>
Reviewed-by: Nicolas Boichat <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/8250/8250_mtk.c

···
 #include <linux/module.h>
 #include <linux/of_irq.h>
 #include <linux/of_platform.h>
+#include <linux/pinctrl/consumer.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
 #include <linux/serial_8250.h>
···
 #ifdef CONFIG_SERIAL_8250_DMA
 	enum dma_rx_status	rx_status;
 #endif
+	int			rx_wakeup_irq;
 };
 
 /* flow control mode */
···
 	pm_runtime_set_active(&pdev->dev);
 	pm_runtime_enable(&pdev->dev);
 
+	data->rx_wakeup_irq = platform_get_irq(pdev, 1);
+
 	return 0;
 }
 
···
 static int __maybe_unused mtk8250_suspend(struct device *dev)
 {
 	struct mtk8250_data *data = dev_get_drvdata(dev);
+	int irq = data->rx_wakeup_irq;
+	int err;
 
 	serial8250_suspend_port(data->line);
+
+	pinctrl_pm_select_sleep_state(dev);
+	if (irq >= 0) {
+		err = enable_irq_wake(irq);
+		if (err) {
+			dev_err(dev,
+				"failed to enable irq wake on IRQ %d: %d\n",
+				irq, err);
+			pinctrl_pm_select_default_state(dev);
+			serial8250_resume_port(data->line);
+			return err;
+		}
+	}
 
 	return 0;
 }
···
 static int __maybe_unused mtk8250_resume(struct device *dev)
 {
 	struct mtk8250_data *data = dev_get_drvdata(dev);
+	int irq = data->rx_wakeup_irq;
+
+	if (irq >= 0)
+		disable_irq_wake(irq);
+	pinctrl_pm_select_default_state(dev);
 
 	serial8250_resume_port(data->line);