Less strict requirements for video device detection

Commit Message

Stefan Bader Aug. 20, 2009, 9:14 a.m. UTC

Hardware: Acer 6920G (from a bug report)

Another case of a broken BIOS. In this case there are several definitions for 
video bus devices but only one has _DOS and _DOD defined. All other definitions 
only have _DOD.
In the past (2.6.27) _ADR was not evaluated to make sure of using a present 
video device, but with that bug brightness could be changed.

Now the video bus having _DOS and _DOD is detected as not being present. The 
other definitions are not considered because they are lacking the _DOS method.
Using the attached patch, would cause the detection code to consider the other 
definitions and has been tested to enable backlight control.

Would this be an acceptable approach? From the ACPI spec it rather sounds like 
_DOD and _DOS must be present for a device for display switching and _DOS would 
indicate possible backlight control as well. So the question might not be so 
much is it the right thing than is it safe enough to allow more compatibility 
with broken implementations without causing other problems...

-Stefan

Comments

Zhang Rui Aug. 21, 2009, 1:17 a.m. UTC | #1

On Thu, 2009-08-20 at 17:14 +0800, Stefan Bader wrote:
> Hardware: Acer 6920G (from a bug report)
> 
> Another case of a broken BIOS. In this case there are several definitions for 
> video bus devices but only one has _DOS and _DOD defined. All other definitions 
> only have _DOD.

I have seen such kind of BIOS too.

> In the past (2.6.27) _ADR was not evaluated to make sure of using a present 
> video device, but with that bug brightness could be changed.
> 
> Now the video bus having _DOS and _DOD is detected as not being present. The 
> other definitions are not considered because they are lacking the _DOS method.
> Using the attached patch, would cause the detection code to consider the other 
> definitions and has been tested to enable backlight control.
> 

> Would this be an acceptable approach?

I think so. I generated a similar patch before, but didn't sent it out
for some reason.
My suggestion is that we should also print out a warning message if _DOS
is missed, what do you think?

thanks,
rui

>  From the ACPI spec it rather sounds like 
> _DOD and _DOS must be present for a device for display switching and _DOS would 
> indicate possible backlight control as well. So the question might not be so 
> much is it the right thing than is it safe enough to allow more compatibility 
> with broken implementations without causing other problems...
> 
> -Stefan
> 

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Patch

From 302f148a5eee5226b15c1dec1dbdfb8326bfa17a Mon Sep 17 00:00:00 2001
From: Stefan Bader <[e-mail]>
Date: Thu, 20 Aug 2009 10:45:58 +0200
Subject: [PATCH] acpi: video: Loosen strictness of video bus detection code

BugLink: http://bugs.launchpad.net/bugs/333386

Currently a video bus device must (beside other criteria) define _DOD and
_DOS methods to be considered a video device.
Some broken BIOSes prevented working backlight control by only defining both
for one (non-existing bus) and only _DOD for the rest. With this patch in
place the other bus definitions were considered too and backlight control
started to work again.

Signed-off-by: Stefan Bader <[e-mail]>
---
 drivers/acpi/video.c        |    2 +-
 drivers/acpi/video_detect.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
index 8851315..f9efeb8 100644
--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -1083,7 +1083,7 @@  static int acpi_video_bus_check(struct acpi_video_bus *video)
 	 */
 
 	/* Does this device support video switching? */
-	if (video->cap._DOS) {
+	if (video->cap._DOS || video->cap._DOD) {
 		video->flags.multihead = 1;
 		status = 0;
 	}
diff --git a/drivers/acpi/video_detect.c b/drivers/acpi/video_detect.c
index 7cd2b63..bee5e34 100644
--- a/drivers/acpi/video_detect.c
+++ b/drivers/acpi/video_detect.c
@@ -82,7 +82,7 @@  long acpi_is_video_device(struct acpi_device *device)
 		return 0;
 
 	/* Does this device able to support video switching ? */
-	if (ACPI_SUCCESS(acpi_get_handle(device->handle, "_DOD", &h_dummy)) &&
+	if (ACPI_SUCCESS(acpi_get_handle(device->handle, "_DOD", &h_dummy)) ||
 	    ACPI_SUCCESS(acpi_get_handle(device->handle, "_DOS", &h_dummy)))
 		video_caps |= ACPI_VIDEO_OUTPUT_SWITCHING;
 
-- 
1.5.4.3