ACPI: video: create "brightness_switch_enabled" modparam

Introduce new module parameter for brightness control.
"brightness_switch_enabled" is set by default which means
nothing changes upon brightness switch events.

When "brightness_switch_enabled" is cleared via
"echo 0 > /sys/module/video/parameters/brightness_switch_enabled",
ACPI will not try to change the brightness level any more.

Either X will take charge of this or users can change the brightness level
by poking /sys/class/backlight/acpi_videoX/...
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/video.c

@@ -72,6 +72,9 @@ MODULE_AUTHOR("Bruno Ducrot");
 MODULE_DESCRIPTION("ACPI Video Driver");
 MODULE_LICENSE("GPL");
 
+static int brightness_switch_enabled = 1;
+module_param(brightness_switch_enabled, bool, 0644);
+
 static int acpi_video_bus_add(struct acpi_device *device);
 static int acpi_video_bus_remove(struct acpi_device *device, int type);
 
@@ -1850,27 +1853,32 @@ static void acpi_video_device_notify(acpi_handle handle, u32 event, void *data)
 
 	switch (event) {
 	case ACPI_VIDEO_NOTIFY_CYCLE_BRIGHTNESS:	/* Cycle brightness */
-		acpi_video_switch_brightness(video_device, event);
+		if (brightness_switch_enabled)
+			acpi_video_switch_brightness(video_device, event);
 		acpi_bus_generate_proc_event(device, event, 0);
 		keycode = KEY_BRIGHTNESS_CYCLE;
 		break;
 	case ACPI_VIDEO_NOTIFY_INC_BRIGHTNESS:	/* Increase brightness */
-		acpi_video_switch_brightness(video_device, event);
+		if (brightness_switch_enabled)
+			acpi_video_switch_brightness(video_device, event);
 		acpi_bus_generate_proc_event(device, event, 0);
 		keycode = KEY_BRIGHTNESSUP;
 		break;
 	case ACPI_VIDEO_NOTIFY_DEC_BRIGHTNESS:	/* Decrease brightness */
-		acpi_video_switch_brightness(video_device, event);
+		if (brightness_switch_enabled)
+			acpi_video_switch_brightness(video_device, event);
 		acpi_bus_generate_proc_event(device, event, 0);
 		keycode = KEY_BRIGHTNESSDOWN;
 		break;
 	case ACPI_VIDEO_NOTIFY_ZERO_BRIGHTNESS:	/* zero brightnesss */
-		acpi_video_switch_brightness(video_device, event);
+		if (brightness_switch_enabled)
+			acpi_video_switch_brightness(video_device, event);
 		acpi_bus_generate_proc_event(device, event, 0);
 		keycode = KEY_BRIGHTNESS_ZERO;
 		break;
 	case ACPI_VIDEO_NOTIFY_DISPLAY_OFF:	/* display device off */
-		acpi_video_switch_brightness(video_device, event);
+		if (brightness_switch_enabled)
+			acpi_video_switch_brightness(video_device, event);
 		acpi_bus_generate_proc_event(device, event, 0);
 		keycode = KEY_DISPLAY_OFF;
 		break;