1.. SPDX-License-Identifier: GPL-2.0
2
3=====================
4ACPI video extensions
5=====================
6
7This driver implement the ACPI Extensions For Display Adapters for
8integrated graphics devices on motherboard, as specified in ACPI 2.0
9Specification, Appendix B, allowing to perform some basic control like
10defining the video POST device, retrieving EDID information or to
11setup a video output, etc.  Note that this is an ref. implementation
12only.  It may or may not work for your integrated video device.
13
14The ACPI video driver does 3 things regarding backlight control.
15
16Export a sysfs interface for user space to control backlight level
17==================================================================
18
19If the ACPI table has a video device, and acpi_backlight=vendor kernel
20command line is not present, the driver will register a backlight device
21and set the required backlight operation structure for it for the sysfs
22interface control. For every registered class device, there will be a
23directory named acpi_videoX under /sys/class/backlight.
24
25The backlight sysfs interface has a standard definition here:
26Documentation/ABI/stable/sysfs-class-backlight.
27
28And what ACPI video driver does is:
29
30actual_brightness:
31  on read, control method _BQC will be evaluated to
32  get the brightness level the firmware thinks it is at;
33bl_power:
34  not implemented, will set the current brightness instead;
35brightness:
36  on write, control method _BCM will run to set the requested brightness level;
37max_brightness:
38  Derived from the _BCL package(see below);
39type:
40  firmware
41
42Note that ACPI video backlight driver will always use index for
43brightness, actual_brightness and max_brightness. So if we have
44the following _BCL package::
45
46	Method (_BCL, 0, NotSerialized)
47	{
48		Return (Package (0x0C)
49		{
50			0x64,
51			0x32,
52			0x0A,
53			0x14,
54			0x1E,
55			0x28,
56			0x32,
57			0x3C,
58			0x46,
59			0x50,
60			0x5A,
61			0x64
62		})
63	}
64
65The first two levels are for when laptop are on AC or on battery and are
66not used by Linux currently. The remaining 10 levels are supported levels
67that we can choose from. The applicable index values are from 0 (that
68corresponds to the 0x0A brightness value) to 9 (that corresponds to the
690x64 brightness value) inclusive. Each of those index values is regarded
70as a "brightness level" indicator. Thus from the user space perspective
71the range of available brightness levels is from 0 to 9 (max_brightness)
72inclusive.
73
74Notify user space about hotkey event
75====================================
76
77There are generally two cases for hotkey event reporting:
78
79i) For some laptops, when user presses the hotkey, a scancode will be
80   generated and sent to user space through the input device created by
81   the keyboard driver as a key type input event, with proper remap, the
82   following key code will appear to user space::
83
84	EV_KEY, KEY_BRIGHTNESSUP
85	EV_KEY, KEY_BRIGHTNESSDOWN
86	etc.
87
88For this case, ACPI video driver does not need to do anything(actually,
89it doesn't even know this happened).
90
91ii) For some laptops, the press of the hotkey will not generate the
92    scancode, instead, firmware will notify the video device ACPI node
93    about the event. The event value is defined in the ACPI spec. ACPI
94    video driver will generate an key type input event according to the
95    notify value it received and send the event to user space through the
96    input device it created:
97
98	=====		==================
99	event		keycode
100	=====		==================
101	0x86		KEY_BRIGHTNESSUP
102	0x87		KEY_BRIGHTNESSDOWN
103	etc.
104	=====		==================
105
106so this would lead to the same effect as case i) now.
107
108Once user space tool receives this event, it can modify the backlight
109level through the sysfs interface.
110
111Change backlight level in the kernel
112====================================
113
114This works for machines covered by case ii) in Section 2. Once the driver
115received a notification, it will set the backlight level accordingly. This does
116not affect the sending of event to user space, they are always sent to user
117space regardless of whether or not the video module controls the backlight level
118directly. This behaviour can be controlled through the brightness_switch_enabled
119module parameter as documented in admin-guide/kernel-parameters.rst. It is
120recommended to disable this behaviour once a GUI environment starts up and
121wants to have full control of the backlight level.
122