1 .. SPDX-License-Identifier: GPL-2.0
2
3 =====================
4 ACPI video extensions
5 =====================
6
7 This driver implement the ACPI Extensions For
8 integrated graphics devices on motherboard, as
9 Specification, Appendix B, allowing to perform
10 defining the video POST device, retrieving EDI
11 setup a video output, etc. Note that this is
12 only. It may or may not work for your integra
13
14 The ACPI video driver does 3 things regarding
15
16 Export a sysfs interface for user space to con
17 ==============================================
18
19 If the ACPI table has a video device, and acpi
20 command line is not present, the driver will r
21 and set the required backlight operation struc
22 interface control. For every registered class
23 directory named acpi_videoX under /sys/class/b
24
25 The backlight sysfs interface has a standard d
26 Documentation/ABI/stable/sysfs-class-backlight
27
28 And what ACPI video driver does is:
29
30 actual_brightness:
31 on read, control method _BQC will be evaluat
32 get the brightness level the firmware thinks
33 bl_power:
34 not implemented, will set the current bright
35 brightness:
36 on write, control method _BCM will run to se
37 max_brightness:
38 Derived from the _BCL package(see below);
39 type:
40 firmware
41
42 Note that ACPI video backlight driver will alw
43 brightness, actual_brightness and max_brightne
44 the 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
65 The first two levels are for when laptop are o
66 not used by Linux currently. The remaining 10
67 that we can choose from. The applicable index
68 corresponds to the 0x0A brightness value) to 9
69 0x64 brightness value) inclusive. Each of thos
70 as a "brightness level" indicator. Thus from t
71 the range of available brightness levels is fr
72 inclusive.
73
74 Notify user space about hotkey event
75 ====================================
76
77 There are generally two cases for hotkey event
78
79 i) For some laptops, when user presses the hot
80 generated and sent to user space through th
81 the keyboard driver as a key type input eve
82 following key code will appear to user spac
83
84 EV_KEY, KEY_BRIGHTNESSUP
85 EV_KEY, KEY_BRIGHTNESSDOWN
86 etc.
87
88 For this case, ACPI video driver does not need
89 it doesn't even know this happened).
90
91 ii) For some laptops, the press of the hotkey
92 scancode, instead, firmware will notify th
93 about the event. The event value is define
94 video driver will generate an key type inp
95 notify value it received and send the even
96 input device it created:
97
98 ===== ==================
99 event keycode
100 ===== ==================
101 0x86 KEY_BRIGHTNESSUP
102 0x87 KEY_BRIGHTNESSDOWN
103 etc.
104 ===== ==================
105
106 so this would lead to the same effect as case
107
108 Once user space tool receives this event, it c
109 level through the sysfs interface.
110
111 Change backlight level in the kernel
112 ====================================
113
114 This works for machines covered by case ii) in
115 received a notification, it will set the backl
116 not affect the sending of event to user space,
117 space regardless of whether or not the video m
118 directly. This behaviour can be controlled thr
119 module parameter as documented in admin-guide/
120 recommended to disable this behaviour once a G
121 wants to have full control of the backlight le
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