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