acpi: tie ACPI backlight devices to PCI devices if possible

Dual-GPU machines may provide more than one ACPI backlight interface.  Tie
the backlight device to the GPU in order to allow userspace to identify
the correct interface.
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Airlie <airlied@linux.ie>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Sedat Dilek <sedat.dilek@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/acpi/video.c

@@ -782,6 +782,9 @@ static void acpi_video_device_find_cap(struct acpi_video_device *device)
 
 	if (acpi_video_backlight_support()) {
 		struct backlight_properties props;
+		struct pci_dev *pdev;
+		acpi_handle acpi_parent;
+		struct device *parent = NULL;
 		int result;
 		static int count = 0;
 		char *name;
@@ -794,10 +797,20 @@ static void acpi_video_device_find_cap(struct acpi_video_device *device)
 			return;
 		count++;
 
+		acpi_get_parent(device->dev->handle, &acpi_parent);
+
+		pdev = acpi_get_pci_dev(acpi_parent);
+		if (pdev) {
+			parent = &pdev->dev;
+			pci_dev_put(pdev);
+		}
+
 		memset(&props, 0, sizeof(struct backlight_properties));
 		props.type = BACKLIGHT_FIRMWARE;
 		props.max_brightness = device->brightness->count - 3;
-		device->backlight = backlight_device_register(name, NULL, device,
+		device->backlight = backlight_device_register(name,
+							      parent,
+							      device,
 							      &acpi_backlight_ops,
 							      &props);
 		kfree(name);