mbp_nvidia_bl: check that the backlight control functions

The SMI-based backlight control functionality may fail to work if the
system is running under EFI rather than BIOS.  Check that the hardware
responds as expected, and exit if it doesn't.
Signed-off-by: NMatthew Garrett <mjg@redhat.com>
Acked-by: NRichard Purdie <richard.purdie@linuxfoundation.org>
Cc: Mourad De Clerck <mourad@aquazul.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/video/backlight/mbp_nvidia_bl.c

@@ -143,6 +143,7 @@ static int __devinit mb_bl_add(struct acpi_device *dev)
 {
 	struct backlight_properties props;
 	struct pci_dev *host;
+	int intensity;
 
 	host = pci_get_bus_and_slot(0, 0);
 
@@ -163,6 +164,18 @@ static int __devinit mb_bl_add(struct acpi_device *dev)
 		return -ENODEV;
 	}
 
+	/* Check that the hardware responds - this may not work under EFI */
+
+	intensity = hw_data->backlight_ops.get_brightness(NULL);
+
+	if (!intensity) {
+		hw_data->set_brightness(1);
+		if (!hw_data->backlight_ops.get_brightness(NULL))
+			return -ENODEV;
+
+		hw_data->set_brightness(0);
+	}
+
 	if (!request_region(hw_data->iostart, hw_data->iolen,
 			    "Macbook backlight"))
 		return -ENXIO;