linux/Documentation/acpi/video_extension.txt
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   1ACPI video extensions
   2~~~~~~~~~~~~~~~~~~~~~
   3
   4This driver implement the ACPI Extensions For Display Adapters for
   5integrated graphics devices on motherboard, as specified in ACPI 2.0
   6Specification, Appendix B, allowing to perform some basic control like
   7defining the video POST device, retrieving EDID information or to
   8setup a video output, etc.  Note that this is an ref. implementation
   9only.  It may or may not work for your integrated video device.
  10
  11The ACPI video driver does 3 things regarding backlight control:
  12
  131 Export a sysfs interface for user space to control backlight level
  14
  15If the ACPI table has a video device, and acpi_backlight=vendor kernel
  16command line is not present, the driver will register a backlight device
  17and set the required backlight operation structure for it for the sysfs
  18interface control. For every registered class device, there will be a
  19directory named acpi_videoX under /sys/class/backlight.
  20
  21The backlight sysfs interface has a standard definition here:
  22Documentation/ABI/stable/sysfs-class-backlight.
  23
  24And what ACPI video driver does is:
  25actual_brightness: on read, control method _BQC will be evaluated to
  26get the brightness level the firmware thinks it is at;
  27bl_power: not implemented, will set the current brightness instead;
  28brightness: on write, control method _BCM will run to set the requested
  29brightness level;
  30max_brightness: Derived from the _BCL package(see below);
  31type: firmware
  32
  33Note that ACPI video backlight driver will always use index for
  34brightness, actual_brightness and max_brightness. So if we have
  35the following _BCL package:
  36
  37Method (_BCL, 0, NotSerialized)
  38{
  39        Return (Package (0x0C)
  40        {
  41                0x64,
  42                0x32,
  43                0x0A,
  44                0x14,
  45                0x1E,
  46                0x28,
  47                0x32,
  48                0x3C,
  49                0x46,
  50                0x50,
  51                0x5A,
  52                0x64
  53        })
  54}
  55
  56The first two levels are for when laptop are on AC or on battery and are
  57not used by Linux currently. The remaining 10 levels are supported levels
  58that we can choose from. The applicable index values are from 0 (that
  59corresponds to the 0x0A brightness value) to 9 (that corresponds to the
  600x64 brightness value) inclusive. Each of those index values is regarded
  61as a "brightness level" indicator. Thus from the user space perspective
  62the range of available brightness levels is from 0 to 9 (max_brightness)
  63inclusive.
  64
  652 Notify user space about hotkey event
  66
  67There are generally two cases for hotkey event reporting:
  68i) For some laptops, when user presses the hotkey, a scancode will be
  69   generated and sent to user space through the input device created by
  70   the keyboard driver as a key type input event, with proper remap, the
  71   following key code will appear to user space:
  72
  73        EV_KEY, KEY_BRIGHTNESSUP
  74        EV_KEY, KEY_BRIGHTNESSDOWN
  75        etc.
  76
  77For this case, ACPI video driver does not need to do anything(actually,
  78it doesn't even know this happened).
  79
  80ii) For some laptops, the press of the hotkey will not generate the
  81    scancode, instead, firmware will notify the video device ACPI node
  82    about the event. The event value is defined in the ACPI spec. ACPI
  83    video driver will generate an key type input event according to the
  84    notify value it received and send the event to user space through the
  85    input device it created:
  86
  87        event           keycode
  88        0x86            KEY_BRIGHTNESSUP
  89        0x87            KEY_BRIGHTNESSDOWN
  90        etc.
  91
  92so this would lead to the same effect as case i) now.
  93
  94Once user space tool receives this event, it can modify the backlight
  95level through the sysfs interface.
  96
  973 Change backlight level in the kernel
  98
  99This works for machines covered by case ii) in Section 2. Once the driver
 100received a notification, it will set the backlight level accordingly. This does
 101not affect the sending of event to user space, they are always sent to user
 102space regardless of whether or not the video module controls the backlight level
 103directly. This behaviour can be controlled through the brightness_switch_enabled
 104module parameter as documented in kernel-parameters.txt. It is recommended to
 105disable this behaviour once a GUI environment starts up and wants to have full
 106control of the backlight level.
 107
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