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# Backlight

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Screen brightness might be tricky to control. On some machines physical hardware switches are missing and software solutions may not work well. However, it is generally possible to find a functional method for a given hardware. This article aims to summarize all possible ways to adjust the backlight.

There are many ways to control brightness of a monitor, laptop or integrated panel (such as the iMac). According to these discussions and this wiki page the control method can be divided into these categories:

- brightness is controlled by vendor-specified hotkey and there is no interface for the OS to adjust the brightness.
- brightness is controlled by either the ACPI, graphic or platform driver. In this case, backlight control is exposed to the user through `/sys/class/backlight` which can be used by user-space backlight utilities.
- brightness is controlled by writing into a graphics card register through setpci.

Note Since OLED screens have no backlight, brightness cannot be controlled by changing backlight power on laptops equipped with an OLED screen. In this case, perceived screen brightness can be adjusted with a PWM control (not implemented in the Linux kernel) or via software color correction.

## Hardware interfaces

### ACPI

The brightness of the screen backlight is adjusted by setting the power level of the backlight LEDs or cathodes. The power level can often be controlled using the ACPI kernel module for video. An interface to this module is provided via a sysfs(5) directory at `/sys/class/backlight/`.

The name of the directory depends on the graphics card model.

    $ ls /sys/class/backlight/

    acpi_video0

In this case, the backlight is managed by an ATI graphics card. In the case of an Intel card, the directory is called `intel_backlight`. In the following examples, `acpi_video0` is used. If you use an Intel card, simply replace `acpi_video0` with `intel_backlight` in the examples.

The directory contains the following files related to brightness:

    $ ls /sys/class/backlight/acpi_video0/*brightness

    actual_brightness  brightness         max_brightness

The maximum brightness can be read from `max_brightness`, like this:

    $ cat /sys/class/backlight/acpi_video0/max_brightness

    15

The brightness can be set by writing a number to `brightness`. However, its value often differs from `actual_brightness`, and it's device dependent. (See also #Recent (2025) AMD changes.)

    # echo 5 > /sys/class/backlight/acpi_video0/brightness

Attempting to set a brightness greater than the maximum results in an error. By default, only `root` can change the brightness by this method. To allow users in the `video` group to change the brightness, a udev rule such as the following can be used (Logging out/Rebooting may be necessary to changes take effects):

    /etc/udev/rules.d/backlight.rules

    ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chgrp video $sys$devpath/brightness", RUN+="/bin/chmod g+w $sys$devpath/brightness"

The factual accuracy of this article or section is disputed.

Reason: Explain why it is not possible to alter file permissions with `GROUP="video", MODE="0664"`. (Discuss in Talk:Backlight#Udev rules for permissions of brightness doesn't work)

#### Kernel command-line options

Sometimes ACPI does not work well due to different motherboard implementations and ACPI quirks. This results in, for instance, inaccurate brightness notifications. This includes some laptops with dual graphics (e.g., NVIDIA/AMD dedicated GPU with Intel/AMD integrated GPU). Additionally, ACPI sometimes needs to register its own `acpi_video0` backlight even if one already exists (such as `intel_backlight`), which can be done by adding one of the following kernel parameters:

    acpi_backlight=video
    acpi_backlight=vendor
    acpi_backlight=native

Tip

- On Nvidia Optimus laptops, the kernel parameter `nomodeset` can interfere with the ability to adjust the backlight.
- On an Asus notebooks you might also need to load the `asus-nb-wmi` kernel module.
- Disabling legacy boot on Dell XPS13 breaks backlight support.
- Since Linux 6.1 the backlight subsystem was revamped, if your backlight does not work after an update first try to remove an existing `acpi_backlight` kernel parameter. On some Optimus laptops, you can try booting with `acpi_backlight=nvidia_wmi_ec`.

#### Udev rule

If the ACPI interface is available, the backlight level can be set at boot using a udev rule:

    /etc/udev/rules.d/81-backlight.rules

    # Set backlight level to 8
    SUBSYSTEM=="backlight", ACTION=="add", KERNEL=="acpi_video0", ATTR{brightness}="8"

Note The systemd-backlight service restores the previous backlight brightness level at boot. To prevent conflicts for the above rules, see #Save and restore functionality.
Tip To set the backlight depending on power state, see Power management#Using a script and a udev rule and use your favourite backlight utility in the script.

### setpci

In some cases (e.g. Intel Mobile 945GME [1]), it is possible to set the register of the graphics card to adjust the backlight. It means you adjust the backlight by manipulating the hardware directly, which can be risky and generally is not a good idea. Not all of the graphics cards support this method.

When using this method, you need to use `lspci` first to find out where your graphics card is.

    # setpci -s 00:02.0 F4.B=0

### External monitors

Display Data Channel Command Interface (DDC/CI) can be used to communicate with external monitors implementing Monitor Control Command Set (MCCS) over I2C. DDC can control brightness, contrast, inputs, etc on supported monitors. Settings available via the On-Screen Display (OSD) panel can usually also be managed via DDC. The kernel module `i2c-dev` may need to be loaded if the `/dev/i2c-*` devices do not exist.

ddcutil can be used to query and set brightness settings:

    # ddcutil capabilities | grep "Feature: 10"

      Feature: 10 (Brightness)

    # ddcutil getvcp 10

    VCP code 0x10 (Brightness                    ): current value =    60, max value =   100

    # ddcutil setvcp 10 70

Alternatively, one may use ddcci-driver-linux-dkmsAUR to expose external monitors in sysfs. Then, after loading the `ddcci` kernel module, one can use any backlight utility.

Note

- Using `ddcci` and `i2c-dev` simultaneously may result in resource conflicts such as a `Device or resource busy` error.
- Users of NVIDIA's proprietary drivers may need to add `Option "RegistryDwords" "RMUseSwI2c=0x01; RMI2cSpeed=100"` to the `Device` section in the Xorg configuration or the `NVreg_RegistryDwords=RMUseSwI2c=0x01;RMI2cSpeed=100` kernel module parameter for the `nvidia` module. See [2] and [3].
- `ddcutil` will fail to set some VCP features if there is a feature enabled on the monitor which already automatically adjusts them (e.g. Dynamic Contrast Ratio or BenQ's Eye Care technology).
- If ddcutil is installed, it provides the `/usr/share/ddcutil/data/90-nvidia-i2c.conf` file, which can be copied to `/etc/X11/xorg.conf.d/` instead of manually editing Xorg configuration files. It also provides `/usr/share/ddcutil/data/60-ddcutil-i2c.rules` and `/usr/share/ddcutil/data/60-ddcutil-usb.rules` for udev rules.

## Switch off the backlight

This article or section is a candidate for merging with DPMS.

Notes: Same topic. (Discuss in Talk:Backlight)

Switching off the backlight (for example when one locks a notebook) can be useful to conserve battery energy. Ideally the following command should work for any Xorg graphical session:

    $ xset dpms force off

The backlight should switch on again on mouse movement or keyboard input. Alternately, `xset s` could be used for a similar effect.

If the previous commands do not work, there is a chance that vbetool may work. Note, however, that in this case the backlight must be manually activated again. The command is as follows:

    $ vbetool dpms off

To activate the backlight again:

    $ vbetool dpms on

For example, this can be put to use when closing the notebook lid using acpid.

## Save and restore functionality

The systemd package includes the service `systemd-backlight@.service`, which is enabled by default and "static". It saves the backlight brightness level at shutdown and restores it at boot. The service uses the ACPI method described in #ACPI, generating services for each folder found in `/sys/class/backlight/`. For example, if there is a folder named `acpi_video0`, it generates a service called `systemd-backlight@backlight:acpi_video0.service`. When using other methods of setting the backlight at boot, it is recommended to stop systemd-backlight from restoring the backlight by setting the kernel parameter `systemd.restore_state=0`. See systemd-backlight@.service(8) for details.

Note Some laptops have multiple video cards (e.g. Optimus) and the backlight restoration fails. Try masking an instance of the service (e.g. `systemd-backlight@backlight:acpi_video1` for `acpi_video1`).

Additionally, the brilloAUR and lightAUR utilities support save and restore functionality. These two may be more useful if one wishes to restore the screen brightness on a per-user basis, however no systemd units are provided to accomplish this.

## Backlight utilities

Note The utilities in the following table can be used to control screen brightness. All of them are compatible with Wayland and do not require X. Some (like brightnessctl or lightAUR) add udev rules to allow members of the `video` (or `input`) group to modify brightness.

Package name | Controls keyboard backlights | Reacts to ambient brightness | Language | License | Notes
--- | --- | --- | --- | --- | ---
acpi-backlight-notifyAUR | No | No | C | MIT | Simple notification daemon for the ACPI interface (reads `/sys/class/backlight/`)
acpilightAUR | Yes | No | Python3 | GPL-3.0-or-later | "xbacklight" executable provided
backlight_controlAUR | No | No | C | MIT | Extremely small and simple. Supports relative adjustments.
blightAUR | Yes | No | Python3 | ISC | Uses logind interface. Restricted to local users, but does not require suid or video group membership.
brightdAUR | No | No | C | GPL-2.0 | Dims the screen when there is no user input for some time.
brightnessctl | Yes | No | C | MIT | -
brilloAUR | Yes | No | C | GPL-3.0-only | Supports smooth and relative adjustments.
clightAUR | Yes | Yes | C | GPL-3.0-or-later | Manages screen temperature (Xorg only) and smoothly dims brightness after a timeout. Supports ambient light sensors [4]. Can turn webcam into an ambient light sensor.
enlighten-gitAUR | Yes | No | C | GPL-3.0-or-later | -
illum-gitAUR | No | No | C | AGPL-3.0 | Reacts to key presses.
lightAUR | Yes | No | C | GPL-3.0-only | Dependency free. Does not rely on X server.
luxAUR | No | No | Shell | MIT | -
macbook-lighterAUR | Yes | Yes | Bash | GPL | Macbook screen/keyboard backlight CLI and auto-adjust on ambient light.
wlumaAUR | Yes | Yes | Rust | ISC | Automatic brightness adjustment based on screen contents and ambient light. Can use webcam or time to simulate ambient light sensor. Supports keyboards and external monitors. Uses wlroots.
xbacklight-notifyAUR | No | No | C | MIT | Simple notification daemon for X11 (reads the RandR backlight property)
ybacklightAUR | No | No | Perl | GPL-2.0 | Small Perl script similar to xbacklight but using sysfs drivers.

Tip Commands involving these utilities can be bound to the `XF86MonBrightnessUp` and `XF86MonBrightnessDown` keyboard keys as described in Keyboard shortcuts#Xorg.

### light

Install lightAUR and add your user to the `video` user group.

Increase backlight brightness by 5 percent:

    $ light -A 5

Decrease backlight brightness by 5 percent:

    $ light -U 5

Set backlight brightness to 100 percent:

    $ light -S 100

### Using DBus with GNOME

Brightness can also be adjusted as the GNOME controls do. Changes are reflected in the GNOME UI using this method.

    $ gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.freedesktop.DBus.Properties.Set org.gnome.SettingsDaemon.Power.Screen Brightness "<int32 50>"

Steps in brightness for keyboard control can be implemented with this method as well.

    $ gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepUp
    $ gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepDown

### Using DBus with KDE

See https://userbase.kde.org/KDE_Connect/Tutorials/Useful_commands#Brightness_settings.

## Color correction

This article or section needs expansion.

Reason: Which utilities require Xorg and which work in Wayland? (Discuss in Talk:Backlight)

Color correction does not change the backlight power, it just modifies the video lookup table: this means that your battery life will be unaffected by the change. Nevertheless, it could be useful when no backlight control is available (desktop PCs or laptops with OLED screens).

- Clight — User daemon utility that aims to fully manage your display. It can manage the screen temperature depending on the current time of the day, just like redshift does. It tries to use geoclue to retrieve the user position if neither latitude or longitude are set in the configuration file. It also supports fixed times for sunrise and sunset.

  https://github.com/FedeDP/Clight || clightAUR

- colord-brightness — Set the brightness via the colord-daemon and ICC-profile under Wayland and Xorg (like icc-brightness but a better performance).

  https://github.com/Nikl174/colord-brightness || colord-brightnessAUR

- Gammastep — Color temperature adjustment tool, a fork of Redshift.

  https://gitlab.com/chinstrap/gammastep || gammastep

- icc-brightness — Control OLED display brightness by applying ICC color profiles.

  https://github.com/udifuchs/icc-brightness || icc-brightness-gnome-gitAUR

- Monica — Monitor calibration tool. It works as frontend to xgamma to alter the gamma correction.

  https://web.archive.org/web/20090815224839/http://www.pcbypaul.com/software/monica.html || monicaAUR

- Redshift — Color temperature adjustment tool. It adjusts the color temperature of your screen according to your surroundings. This may help your eyes hurt less if you are working in front of the screen at night. This program is inspired by f.lux.

  http://jonls.dk/redshift/ || redshift

- xcalib — Lightweight monitor calibration loader which can load an ICC monitor profile to be shared across desktop applications.

  https://github.com/OpenICC/xcalib || xcalib

- xgamma — Alter a monitor's gamma correction.

  https://xorg.freedesktop.org/ || xorg-xgamma

### Wayland

Redshift does not support Wayland (although a patchset exists). But it is possible to apply the desired temperature in tty before starting a compositor. For example:

    $ redshift -m drm -PO 3000

Otherwise some compositors can apply color correction during runtime:

- On GNOME, the built-in Night Light can be used.
- On KDE Plasma, the built-in KDE#Night Light can be used.
- On Sway and other wlroots-based compositors, as well as Orbital, Redshift fork gammastep, wlsunset, clightAUR, or wl-gammarelayAUR can be used.

### Xorg: adjust perceived brightness with xrandr

xrandr may be used to adjust the perceived brightness.

To adjust perceived brightness above its maximum level (the same caveats mentioned above for Nvidia apply):

    $ xrandr --output output_name --brightness 2

This should roughly double luma in the image. It will sacrifice color quality for brightness, nevertheless it is particularly suited for situations where the ambient light is very bright (e.g. sunlight).

This can also be used to reduce perceived brightness in a dark room by specifying some value less than 1 (e.g. 0.5), this is useful when no backlight control is available (e.g. desktop PC).

The output name of the connected device may be determined by calling `xrandr`:

    $ xrandr | grep -w connected | cut -f '1' -d ' '

Users may find it convenient to implement this as an alias:

    $ alias b='echo -e "enter brightness:\n"; read val; xrandr --output output name --brightness "${val}"'

To automatically call xrandr when a backlight file changes, oled_shmoledAUR can be used like so:

    $ oled_shmoled output_name

### NVIDIA settings

Users of NVIDIA's proprietary drivers can change display brightness via the nvidia-settings utility under "X Server Color Correction." However, note that this has absolutely nothing to do with backlight (intensity), it merely adjusts the color output. (Reducing brightness this way is a power-inefficient last resort when all other options fail; increasing brightness spoils your color output completely, in a way similar to overexposed photos.)

## Troubleshooting

### Backlight PWM modulation frequency (Intel i915 only)

This article or section needs expansion.

Reason: Modern LED display using IPS panel usually utilize DC dimming instead of PWM dimming. This can be indicated by the two upper bytes of 0xC8254 register 0x0001, which means the frequency is almost infinity. (Discuss in Talk:Backlight)

Laptops with LED backlight are known to have screen flicker sometimes. This is because the most efficient way of controlling LED backlight brightness is by turning the LED's on and off very quickly varying the amount of time they are on.

However, the frequency of the switching, so-called PWM (pulse-width modulation) frequency, may not be high enough for the eye to perceive it as a single brightness and instead see flickering. This causes some people to have symptoms such as headaches and eyestrain.

If you have an Intel i915 GPU, then it may be possible to adjust PWM frequency to eliminate flicker.

Period of PWM (inverse to frequency) is stored in 2 higher bytes of `0xC8254` register (if you are using the Intel GM45 chipset use address `0x61254` instead). To manipulate registers values, install the intel-gpu-tools package.

To increase the frequency, period must be reduced. For example:

    # intel_reg read 0xC8254

    0xC8254 : 0x12281228

Then to double PWM frequency divide 2 higher bytes (4 higher hex digits) by 2 and write back resulting value, keeping lower bytes unchanged:

    # intel_reg write 0xC8254 0x09141228

You can use online calculator to calculate desired value https://devbraindom.blogspot.com/2013/03/eliminate-led-screen-flicker-with-intel.html[dead link 2025-03-15—HTTP 404]

To set new frequency automatically, consider writing an udev rule or install intelpwm-udevAUR.

### Inverted Brightness (Intel i915 only)

Symptoms:

- after installing xf86-video-intel systemd-backlight.service turns off the backlight during boot
  - possible solution: mask systemd-backlight.service
- switching from X to another VT turns the backlight off
- the brightness keys are inverted (i.e. turning up the brightness makes the screen darker)

This problem may be solved by adding `i915.invert_brightness=1` to the list of kernel parameters.

### Unable to control eDP Panel brightness (Intel i915 only)

Embedded Display Port (eDP) v1.2 introduced a new display panel control protocol for backlight and other controls that works through the AUX channel [5]

By default the i915 driver tries to use PWM to control backlight brightness, which might not work.

To set the backlight through writes to DPCD registers using the AUX channel set `i915.enable_dpcd_backlight=1` as a kernel parameter.

### sysfs modified but no brightness change

Note This behavior and their workarounds have been confirmed on the Dell M6700 with Nvidia K5000m (BIOS version prior to A10) and Clevo P750ZM (Eurocom P5 Pro Extreme) with Nvidia 980m.

On some systems, the brightness hotkeys on your keyboard correctly modify the values of the acpi interface in `/sys/class/backlight/acpi_video0/actual_brightness` but the brightness of the screen is not changed. Brightness applets from desktop environments may also show changes to no effect.

If you have tested the recommended kernel parameters and only `xbacklight` works, then you may be facing an incompatibility between your BIOS and kernel driver.

In this case the only solution is to wait for a fix either from the BIOS or GPU driver manufacturer.

A workaround is to use the inotify kernel api to trigger `xbacklight` each time the value of `/sys/class/backlight/acpi_video0/actual_brightness` changes.

First install inotify-tools. Then create a script around inotify that will be launched upon each boot or through autostart.

    /usr/local/bin/xbacklightmon

    #!/bin/sh

    path=/sys/class/backlight/acpi_video0

    luminance() {
        read -r level < "$path"/actual_brightness
        factor=$(printf "$max" | awk '{print 100/$1}')
        printf "$level $factor" | awk '{print int($1*$2)}'
    }

    read -r max < "$path"/max_brightness

    xbacklight -set "$(luminance)"

    inotifywait -me modify --format '' "$path"/actual_brightness | while read; do
        xbacklight -set "$(luminance)"
    done

### NVIDIA-only laptops with nvidia_wmi_ec_backlight

On laptops where the Intel iGPU is disabled (e.g. via a MUX switch), such as the Razer Blade 18 (2023, RZ09-0484) with i9-13950HX and RTX 4090, the `nvidia_wmi_ec_backlight` driver registers a backlight device under `/sys/class/backlight/` and accepts brightness values, but the panel brightness does not actually change. The kernel log may show:

    nvidia-modeset: ACPI reported no NVIDIA native backlight available; attempting to use ACPI backlight.

Adding `acpi_backlight=native` to the kernel parameters resolves the issue, allowing `nvidia_wmi_ec_backlight` to properly control the panel backlight.

Note Using `acpi_backlight=vendor` may cause the backlight interface to disappear entirely on these systems.

### sysfs and xbacklight both not working

check dmesg if you have seen like this :

`i915 0000:00:02.0: [drm] *ERROR* [CONNECTOR:114:DSI-1] Failed to get the SoC PWM chip`

Change `/etc/mkinitcpio.conf` to match the following:

    /etc/mkinitcpio.conf

    ...
    MODULES=(i915 pwm-lpss-platform)
    ...

Then regenerate the initramfs.

### Backlight not working in MATE

Make sure the mate-power-manager package is installed.

### Backlight control with function keys not working on Notebooks

Some Notebook Models e.g. Razer Blade 14, Lenovo Yoga Slim 7, Lenovo IdeaPad Gaming 3 and Acer AN517-41 have issues with backlight control, pass `acpi_backlight=video` and/or `amdgpu.backlight=0` as kernel parameters.

### Backlight keys not working in Xfce

In xfce4, the Xfce4 Power Manager handles the brightness keys.

In some installations of Xfce, the "Handle display brightness keys" setting may be turned off by default.

To activate the brightness keys again, open the Xfce Power Manager dialog and toggle on "Handle display brightness keys":

    $ xfce4-power-manager -c

### xbacklight returns : No outputs have backlight property

Depending on the video card installed, sometimes xbacklight from xorg-xbacklight returns the message "No outputs have backlight property". Installing acpilightAUR provides an alternative xbacklight that may work as expected.

### Backlight is always at full brightness after a reboot with amdgpu driver

This article or section is being considered for removal.

Reason: Looks like this has been fixed. (Discuss in Talk:Backlight)

Due to a bug in the amdgpu driver, the backlight's `actual_brightness` value is reported as a 16-bit integer, which is outside the 8-bit range specified in `max_brightness`. This causes the `systemd-backlight` service to attempt to restore, at boot time, a value that is too large and ends being truncated to maximum brightness (255).

While the bug is not addressed, one possible workaround is to modify the stored brightness to within the correct range before it is restored. This can be accomplished with a script and a service unit:

    fix-brightness.sh

    #!/bin/bash

    # Change the line below according to your hardware
    BRIGHTNESS_FILE="/var/lib/systemd/backlight/pci-0000:04:00.0:backlight:amdgpu_bl0"
    BRIGHTNESS=$(cat "$BRIGHTNESS_FILE")
    BRIGHTNESS=$(($BRIGHTNESS*255/65535))
    BRIGHTNESS=${BRIGHTNESS/.*} # truncating to int, just in case
    echo $BRIGHTNESS > "$BRIGHTNESS_FILE"

    fix-brightness.service

    [Unit]
    Description=Convert 16-bit brightness values to 8-bit before systemd-backlight applies it
    Before=systemd-backlight@backlight:amdgpu_bl0.service

    [Service]
    Type=oneshot
    ExecStart=/path/to/fix-brightness.sh

    [Install]
    WantedBy=multi-user.target

On certain systems, the backlight level reported by the driver is in the correct range [0, 255], but systemd still fails to restore the correct value. This is probably due to a race in the kernel. In this case, truncating the brightness level will not help since it is already in the correct range. Instead, saving the brightness level to systemd before shutting down could work as a workaround. This can be accomplished by the following script and service unit:

    fix-brightness.sh

    #!/bin/sh

    # Backlight level from systemd's perspective (change if needed)
    readonly SYSTEMD_BACKLIGHT_FILE='/var/lib/systemd/backlight/pci-0000:04:00.0:backlight:amdgpu_bl0'

    # Backlight level from AMDGPU driver
    readonly AMDGPU_BACKLIGHT_FILE='/sys/class/backlight/amdgpu_bl0/brightness'

    # Read current value from the driver and apply it to systemd
    readonly AMDGPU_BACKLIGHT_VALUE=$(cat "$AMDGPU_BACKLIGHT_FILE")
    echo "$AMDGPU_BACKLIGHT_VALUE" > "$SYSTEMD_BACKLIGHT_FILE"

    fix-brightness.service

    [Unit]
    Description=Save brightness value from AMDGPU
    DefaultDependencies=no
    After=final.target

    [Service]
    Type=oneshot
    ExecStart=/path/to/fix-brightness.sh

    [Install]
    WantedBy=final.target

### Allow <5% brightness to persist on reboot (disable backlight clamp)

According to systemd-backlight@.service(8), if the udev property `ID_BACKLIGHT_CLAMP` is not set to false, the brightness is clamped to a value of at least 1 or 5% of maximum brightness, whichever is greater. This restriction will be removed when the kernel allows user space to reliably set a brightness value which does not turn off the display.

To allow <5% brightness to persist on reboot, create udev rule:

    /etc/udev/rules.d/99-backlight_clamp.rules

    # Allow <5% brightness to persist on reboot (disable clamped value of 5%)
    SUBSYSTEM=="backlight", ENV{ID_BACKLIGHT_CLAMP}="0"

### Recent (2025) AMD changes

This article or section is being considered for removal.

Reason: 6.18 is our LTS now. (Discuss in Talk:Backlight)

In 2025 there were two changes for AMD laptops, affecting kernel 6.16 and backported to 6.15.5. First, the previous max brightness was <= 255, but it is now <= 65535. [6] Notice this means using backlight values defined before this change results in a lower brightness.

Second is the relation of `brightness` and `actual_brightness`, found under `/sys/class/backlight/amdgpu_bl0` or so. Both in the former and the new kernels, these two values are not identical. In the new behavior, actual brightness changes according to the firmware setting. [7] [8] This is because the brightness curve in the firmware possibly better reflects the characteristics of the panel.

To disable this firmware curve, set the following kernel parameter: amdgpu.dcdebugmask=0x40000

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- Laptops

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