3.5.0-rc2 bisected regression: Can't change brightness on Vaio laptop

[Carlos R. Mafra]

Commit ea9f8856bd6d4 ("ACPI video: Harden video bus adding") breaks the
backlight controls on my Vaio laptop.

After this patch the folder /sys/class/backlight becomes empty and
I can't change the lcd brightness anymore.

This regression is present in v3.4.2 as well as in the current
v3.5.0-rc2. 

I tried to manually revert that commit from v3.5.0-rc2, and the resulting
patch below fixes the issue for me.

My config is here:
http://www.damtp.cam.ac.uk/user/crm66/config-3.5-rc1

and the dmesg of v3.5.0-rc2 plus the patch below is here:
http://www.damtp.cam.ac.uk/user/crm66/dmesg_v3.5-rc2+patch.txt

diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
index a576575..4612b1c 100644
--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -548,27 +548,27 @@ acpi_video_device_EDID(struct acpi_video_device *device,
  *		1. 	The system BIOS should NOT automatically control the brightness 
  *			level of the LCD when the power changes from AC to DC.
  * Return Value:
- *		-EINVAL	wrong arg.
+ * 		-1	wrong arg.
  */
 
 static int
 acpi_video_bus_DOS(struct acpi_video_bus *video, int bios_flag, int lcd_flag)
 {
-	acpi_status status;
+	u64 status = 0;
 	union acpi_object arg0 = { ACPI_TYPE_INTEGER };
 	struct acpi_object_list args = { 1, &arg0 };
 
 
-	if (bios_flag < 0 || bios_flag > 3 || lcd_flag < 0 || lcd_flag > 1)
-		return -EINVAL;
+	if (bios_flag < 0 || bios_flag > 3 || lcd_flag < 0 || lcd_flag > 1) {
+		status = -1;
+		goto Failed;
+	}
 	arg0.integer.value = (lcd_flag << 2) | bios_flag;
 	video->dos_setting = arg0.integer.value;
-	status = acpi_evaluate_object(video->device->handle, "_DOS",
-		&args, NULL);
-	if (ACPI_FAILURE(status))
-		return -EIO;
+	acpi_evaluate_object(video->device->handle, "_DOS", &args, NULL);
 
-	return 0;
+      Failed:
+	return status;
 }
 
 /*
@@ -1343,17 +1343,15 @@ static int
 acpi_video_bus_get_devices(struct acpi_video_bus *video,
 			   struct acpi_device *device)
 {
-	int status;
+	int status = 0;
 	struct acpi_device *dev;
 
-	status = acpi_video_device_enumerate(video);
-	if (status)
-		return status;
+	acpi_video_device_enumerate(video);
 
 	list_for_each_entry(dev, &device->children, node) {
 
 		status = acpi_video_bus_get_one_device(dev, video);
-		if (status) {
+		if (ACPI_FAILURE(status)) {
 			printk(KERN_WARNING PREFIX
 					"Can't attach device\n");
 			continue;
@@ -1655,9 +1653,8 @@ static int acpi_video_bus_add(struct acpi_device *device)
 	mutex_init(&video->device_list_lock);
 	INIT_LIST_HEAD(&video->video_device_list);
 
-	error = acpi_video_bus_get_devices(video, device);
-	if (error)
-		goto err_free_video;
+	acpi_video_bus_get_devices(video, device);
+	acpi_video_bus_start_devices(video);
 
 	video->input = input = input_allocate_device();
 	if (!input) {
@@ -1695,13 +1692,9 @@ static int acpi_video_bus_add(struct acpi_device *device)
 
 	video->pm_nb.notifier_call = acpi_video_resume;
 	video->pm_nb.priority = 0;
-	error = register_pm_notifier(&video->pm_nb);
-	if (error)
-		goto err_stop_video;
 
-	error = input_register_device(input);
-	if (error)
-		goto err_unregister_pm_notifier;
+	register_pm_notifier(&video->pm_nb);
+	input_register_device(input);
 
 	return 0;
 

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