mirror of
https://github.com/FEX-Emu/linux.git
synced 2024-12-27 03:47:43 +00:00
86393865f0
The ACPI video driver has changed a lot, and it doesn't export interfaces in /proc any more, so the documentation for it should be updated. This update focuses on ACPI video driver's backlight control. Signed-off-by: Aaron Lu <aaron.lu@intel.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
107 lines
4.0 KiB
Plaintext
107 lines
4.0 KiB
Plaintext
ACPI video extensions
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
This driver implement the ACPI Extensions For Display Adapters for
|
|
integrated graphics devices on motherboard, as specified in ACPI 2.0
|
|
Specification, Appendix B, allowing to perform some basic control like
|
|
defining the video POST device, retrieving EDID information or to
|
|
setup a video output, etc. Note that this is an ref. implementation
|
|
only. It may or may not work for your integrated video device.
|
|
|
|
The ACPI video driver does 3 things regarding backlight control:
|
|
|
|
1 Export a sysfs interface for user space to control backlight level
|
|
|
|
If the ACPI table has a video device, and acpi_backlight=vendor kernel
|
|
command line is not present, the driver will register a backlight device
|
|
and set the required backlight operation structure for it for the sysfs
|
|
interface control. For every registered class device, there will be a
|
|
directory named acpi_videoX under /sys/class/backlight.
|
|
|
|
The backlight sysfs interface has a standard definition here:
|
|
Documentation/ABI/stable/sysfs-class-backlight.
|
|
|
|
And what ACPI video driver does is:
|
|
actual_brightness: on read, control method _BQC will be evaluated to
|
|
get the brightness level the firmware thinks it is at;
|
|
bl_power: not implemented, will set the current brightness instead;
|
|
brightness: on write, control method _BCM will run to set the requested
|
|
brightness level;
|
|
max_brightness: Derived from the _BCL package(see below);
|
|
type: firmware
|
|
|
|
Note that ACPI video backlight driver will always use index for
|
|
brightness, actual_brightness and max_brightness. So if we have
|
|
the following _BCL package:
|
|
|
|
Method (_BCL, 0, NotSerialized)
|
|
{
|
|
Return (Package (0x0C)
|
|
{
|
|
0x64,
|
|
0x32,
|
|
0x0A,
|
|
0x14,
|
|
0x1E,
|
|
0x28,
|
|
0x32,
|
|
0x3C,
|
|
0x46,
|
|
0x50,
|
|
0x5A,
|
|
0x64
|
|
})
|
|
}
|
|
|
|
The first two levels are for when laptop are on AC or on battery and are
|
|
not used by Linux currently. The remaining 10 levels are supported levels
|
|
that we can choose from. The applicable index values are from 0 (that
|
|
corresponds to the 0x0A brightness value) to 9 (that corresponds to the
|
|
0x64 brightness value) inclusive. Each of those index values is regarded
|
|
as a "brightness level" indicator. Thus from the user space perspective
|
|
the range of available brightness levels is from 0 to 9 (max_brightness)
|
|
inclusive.
|
|
|
|
2 Notify user space about hotkey event
|
|
|
|
There are generally two cases for hotkey event reporting:
|
|
i) For some laptops, when user presses the hotkey, a scancode will be
|
|
generated and sent to user space through the input device created by
|
|
the keyboard driver as a key type input event, with proper remap, the
|
|
following key code will appear to user space:
|
|
|
|
EV_KEY, KEY_BRIGHTNESSUP
|
|
EV_KEY, KEY_BRIGHTNESSDOWN
|
|
etc.
|
|
|
|
For this case, ACPI video driver does not need to do anything(actually,
|
|
it doesn't even know this happened).
|
|
|
|
ii) For some laptops, the press of the hotkey will not generate the
|
|
scancode, instead, firmware will notify the video device ACPI node
|
|
about the event. The event value is defined in the ACPI spec. ACPI
|
|
video driver will generate an key type input event according to the
|
|
notify value it received and send the event to user space through the
|
|
input device it created:
|
|
|
|
event keycode
|
|
0x86 KEY_BRIGHTNESSUP
|
|
0x87 KEY_BRIGHTNESSDOWN
|
|
etc.
|
|
|
|
so this would lead to the same effect as case i) now.
|
|
|
|
Once user space tool receives this event, it can modify the backlight
|
|
level through the sysfs interface.
|
|
|
|
3 Change backlight level in the kernel
|
|
|
|
This works for machines covered by case ii) in Section 2. Once the driver
|
|
received a notification, it will set the backlight level accordingly. This does
|
|
not affect the sending of event to user space, they are always sent to user
|
|
space regardless of whether or not the video module controls the backlight level
|
|
directly. This behaviour can be controlled through the brightness_switch_enabled
|
|
module parameter as documented in kernel-parameters.txt. It is recommended to
|
|
disable this behaviour once a GUI environment starts up and wants to have full
|
|
control of the backlight level.
|