Still use ACPI backlight control if _DOS doesn't exist

[Dave Airlie <airlied@xxxxxxxxxx>]

This is a regression introduced by commit
ea9f8856bd6d4ed45885b06a338f7362cd6c60e5

https://bugzilla.kernel.org/show_bug.cgi?id=43168

Some platforms don't have _DOS control method, but the ACPI
backlight still works.
We do not want to lose the backlight control ability on these platforms.

[airlied: this is breaking nouveau really badly on 3.4 and after and there is
no movement on sending it upstream, so please apply]

Cc: Igor Murzov <intergalactic.anonymous@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: len.brown@xxxxxxxxx
Signed-off-by: Zhang Rui <rui.zhang@xxxxxxxxx>
---
 drivers/acpi/video.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Index: rtd3/drivers/acpi/video.c
===================================================================
--- rtd3.orig/drivers/acpi/video.c
+++ rtd3/drivers/acpi/video.c
@@ -565,8 +565,14 @@ acpi_video_bus_DOS(struct acpi_video_bus
 	video->dos_setting = arg0.integer.value;
 	status = acpi_evaluate_object(video->device->handle, "_DOS",
 		&args, NULL);
-	if (ACPI_FAILURE(status))
-		return -EIO;
+	if (ACPI_FAILURE(status)) {
+		/*
+		 * some platforms don't have _DOS, but the ACPI
+		 * backlight control still works
+		 */
+		ACPI_DEBUG_PRINT((ACPI_DB_INFO, "No _DOS\n"));
+		return 0;
+	}
 
 	return 0;
 }
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