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diff --git a/Documentation/firmware-guide/acpi/video_extension.rst b/Documentation/firmware-guide/acpi/video_extension.rst new file mode 100644 index 000000000000..099b8607e07b --- /dev/null +++ b/Documentation/firmware-guide/acpi/video_extension.rst @@ -0,0 +1,121 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +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. + +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. + +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. + +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 admin-guide/kernel-parameters.rst. It is +recommended to disable this behaviour once a GUI environment starts up and +wants to have full control of the backlight level. |