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