set /proc/acpi/video/*/DOS to 4 by default instead of 0

acpi_video_bus_start_devices() in drivers/acpi/video.c sets the default ACPI DOS value to 0, which lets the OS handle toggling of display output but still leaves the BIOS handling brightness levels automatically when connecting/disconnecting AC.

We want the OS to handle both where possible, so a better default would be to set this to 4, which can be done with a one-liner change:

- return acpi_video_bus_DOS(video, 0, 0);
+ return acpi_video_bus_DOS(video, 0, 1);

The one problem with doing this is that, if the system uses proprietary video drivers, we don't have this brightness control so we need to let the BIOS do it instead. But the kernel default should be optimized for the common case, and we should reset this from 4 to 0 only when the proprietary video drivers need us to (via a udev rule, in the X driver's packaging, etc).

In jaunty and earlier, this bit was set by default for intel and ati chips via the hotkey-setup package, which is now obsolete; so I believe that technically there's a regression here in karmic - but fixing that without breaking other systems that already work as intended would be non-trivial, so we should just fix this in lucid instead.

Matthew Garrett explains the rationale for originally setting this only on intel and ati:

<mjg59> The reason hotkey-setup did that check was because only a subset of drivers supported xrandr 1.2
<mjg59> And for drivers that didn't, it was arguably better to leave it under firmware control than to send the event

<mjg59> In Fedora we've just ended up patching the kernel
<mjg59> In principle that may reduce the functionality on some hardware
<mjg59> But only really stuff running closed drivers

<mjg59> We use 4 by default

ProblemType: Bug
Architecture: amd64
ArecordDevices:
 **** List of CAPTURE Hardware Devices ****
 card 0: Intel [HDA Intel], device 0: AD198x Analog [AD198x Analog]
   Subdevices: 1/1
   Subdevice #0: subdevice #0
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: vorlon 6116 F.... pulseaudio
 /dev/snd/pcmC0D0p: vorlon 6116 F...m pulseaudio
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xee240000 irq 17'
   Mixer name : 'Analog Devices AD1981'
   Components : 'HDA:11d41981,17aa2025,00100200'
   Controls : 20
   Simple ctrls : 11
Date: Fri Oct 23 03:56:00 2009
DistroRelease: Ubuntu 9.10
HibernationDevice: RESUME=UUID=f108133c-6b9d-4d28-9058-0b3a0c5549b4
Lsusb:
 Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
MachineType: LENOVO 6371CTO
Package: linux-image-2.6.31-14-generic 2.6.31-14.48
PccardctlIdent:
 Socket 0:
   no product info available
PccardctlStatus:
 Socket 0:
   no card
ProcCmdLine: root=/dev/mapper/hostname-root ro quiet splash
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-14.48-generic
RelatedPackageVersions:
 linux-backports-modules-2.6.31-14-generic N/A
 linux-firmware 1.24
SourcePackage: linux
Uname: Linux 2.6.31-14-generic x86_64
WpaSupplicantLog:

dmi.bios.date: 12/27/2006
dmi.bios.vendor: LENOVO
dmi.bios.version: 7IET23WW (1.04 )
dmi.board.name: 6371CTO
dmi.board.vendor: LENOVO
dmi.board.version: Not Available
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Not Available
dmi.modalias: dmi:bvnLENOVO:bvr7IET23WW(1.04):bd12/27/2006:svnLENOVO:pn6371CTO:pvrThinkPadT60:rvnLENOVO:rn6371CTO:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.name: 6371CTO
dmi.product.version: ThinkPad T60
dmi.sys.vendor: LENOVO