platform/x86: intel-vbtn: Support tablet mode switch

On some laptop like the Dell Inspiron 7000 series tablet mode switch
implemented in Intel ACPI, the events to enter and exit the tablet mode
are 0xCC and 0xCD

This initializes the tablet/laptop mode at the correct value
if the system booted in tablet mode (or the intel-vbtn module
loaded with the device in tablet mode)

Cc: platform-driver-x86@vger.kernel.org
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: "Pali Rohár" <pali.rohar@gmail.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Mario Limonciello <mario_limonciello@dell.com>
Cc: Andy Shevchenko <andy@infradead.org>
Cc: Stefan Brüns<stefan.bruens@rwth-aachen.de>
Signed-off-by: Marco Martin <notmart@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@dell.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
[andy: fixed style of comments, indentation, and massaged commit message]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/intel-vbtn.c

@@ -26,6 +26,9 @@
 #include <linux/suspend.h>
 #include <acpi/acpi_bus.h>
 
+/* When NOT in tablet mode, VGBS returns with the flag 0x40 */
+#define TABLET_MODE_FLAG 0x40
+
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("AceLan Kao");
 
@@ -108,6 +111,7 @@ static void notify_handler(acpi_handle handle, u32 event, void *context)
 
 static int intel_vbtn_probe(struct platform_device *device)
 {
+	struct acpi_buffer vgbs_output = { ACPI_ALLOCATE_BUFFER, NULL };
 	acpi_handle handle = ACPI_HANDLE(&device->dev);
 	struct intel_vbtn_priv *priv;
 	acpi_status status;
@@ -130,6 +134,23 @@ static int intel_vbtn_probe(struct platform_device *device)
 		return err;
 	}
 
+	/*
+	 * VGBS being present and returning something means we have
+	 * a tablet mode switch.
+	 */
+	status = acpi_evaluate_object(handle, "VGBS", NULL, &vgbs_output);
+	if (ACPI_SUCCESS(status)) {
+		union acpi_object *obj = vgbs_output.pointer;
+
+		if (obj && obj->type == ACPI_TYPE_INTEGER) {
+			int m = !(obj->integer.value & TABLET_MODE_FLAG);
+
+			input_report_switch(priv->input_dev, SW_TABLET_MODE, m);
+		}
+	}
+
+	kfree(vgbs_output.pointer);
+
 	status = acpi_install_notify_handler(handle,
 					     ACPI_DEVICE_NOTIFY,
 					     notify_handler,