According to the ACPI spec, "_DOS method is required if the system
supports display switching or LCD brightness control."

But some buggy BIOSes don't export _DOS method while it provides a
set of working backlight control methods.
http://bugzilla.kernel.org/show_bug.cgi?id=13577

With this patch applied, ACPI video driver only gives a warning
message if _DOS is missed, and continue to support the backlight
control without touching _DOS method. 

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
---
 drivers/acpi/video.c        |   18 +++++++++++-------
 drivers/acpi/video_detect.c |    8 ++++++--
 2 files changed, 17 insertions(+), 9 deletions(-)

Index: linux-2.6/drivers/acpi/video.c
===================================================================
--- linux-2.6.orig/drivers/acpi/video.c
+++ linux-2.6/drivers/acpi/video.c
@@ -1069,10 +1069,10 @@ static int acpi_video_bus_check(struct a
 	 */
 
 	/* Does this device support video switching? */
-	if (video->cap._DOS) {
-		video->flags.multihead = 1;
+	if (video->cap._DOD)
 		status = 0;
-	}
+	if (video->cap._DOS)
+		video->flags.multihead = 1;
 
 	/* Does this device support retrieving a video ROM? */
 	if (video->cap._ROM) {
@@ -1488,8 +1488,10 @@ static int acpi_video_bus_DOS_seq_show(s
 {
 	struct acpi_video_bus *video = seq->private;
 
-
-	seq_printf(seq, "DOS setting: <%d>\n", video->dos_setting);
+	if (video->cap._DOS)
+		seq_printf(seq, "DOS setting: <%d>\n", video->dos_setting);
+	else
+		seq_printf(seq, "N/A\n");
 
 	return 0;
 }
@@ -1556,6 +1558,8 @@ acpi_video_bus_write_DOS(struct file *fi
 	char str[12] = { 0 };
 	unsigned long opt;
 
+	if (!video->cap._DOS)
+		return -EINVAL;
 
 	if (!video || count + 1 > sizeof str)
 		return -EINVAL;
@@ -2035,12 +2039,12 @@ static int acpi_video_bus_put_devices(st
 
 static int acpi_video_bus_start_devices(struct acpi_video_bus *video)
 {
-	return acpi_video_bus_DOS(video, 0, 0);
+	return video->cap._DOS ? acpi_video_bus_DOS(video, 0, 0) : 0;
 }
 
 static int acpi_video_bus_stop_devices(struct acpi_video_bus *video)
 {
-	return acpi_video_bus_DOS(video, 0, 1);
+	return video->cap._DOS ? acpi_video_bus_DOS(video, 0, 1) : 0;
 }
 
 static void acpi_video_bus_notify(struct acpi_device *device, u32 event)
Index: linux-2.6/drivers/acpi/video_detect.c
===================================================================
--- linux-2.6.orig/drivers/acpi/video_detect.c
+++ linux-2.6/drivers/acpi/video_detect.c
@@ -81,9 +81,13 @@ long acpi_is_video_device(struct acpi_de
 		return 0;
 
 	/* Does this device able to support video switching ? */
-	if (ACPI_SUCCESS(acpi_get_handle(device->handle, "_DOD", &h_dummy)) &&
-	    ACPI_SUCCESS(acpi_get_handle(device->handle, "_DOS", &h_dummy)))
+	if (ACPI_SUCCESS(acpi_get_handle(device->handle, "_DOD", &h_dummy))) {
+		if (ACPI_FAILURE(acpi_get_handle(device->handle, "_DOS",
+						 &h_dummy)))
+			printk(KERN_WARNING FW_BUG PREFIX "ACPI video bus "
+					"device misses _DOS function\n");
 		video_caps |= ACPI_VIDEO_OUTPUT_SWITCHING;
+	}
 
 	/* Does this device able to retrieve a video ROM ? */
 	if (ACPI_SUCCESS(acpi_get_handle(device->handle, "_ROM", &h_dummy)))