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Architecture discussion for brightness control in KWin

As part of my SoK project, I am creating this issue to discuss the proposed architecture for implementing brightness control in KWin

Current Status

The current architecture looks like this:

graph TB

  Action -- "DBus" --> Kact
  Enum -- "DBus" --> plasma-workspace/applets/brightness
  
  subgraph plasma-workspace/applets/brightness
  Action[User Action]
  end
  
  subgraph powerdevil/daemon
  Kact("KAuth Action (backlightbrightness.cpp)")
  Enum("Screen Enumeration (backlighthelper_linux.cpp)")

end

Proposals

There are a few different ways we could implement brightness control in KWin:

Wayland only (1)

If X11 support can be dropped, we could move the brightness handling code to kwin completely. the architecture could look like this:

graph TB

  Action -- "DBus/Wayland API" --> HDR
  Enum -- "DBus/Wayland API" --> plasma-workspace/applets/brightness
  HDR --> Kact

  subgraph plasma-workspace/applets/brightness
  Action[User Action]
  end
  
  subgraph kwin/new_code
  HDR("HDR handling")
  Kact("KAuth/logind/sysfs call")
  Enum("Screen Enumeration")

end

Benefits

  • Simple architecture
  • No code duplication

Drawbacks

  • No X11 support
  • More code changes required

Wayland + X11 (2)

If X11 support needs to be retained, we need to support the scenario where a different window manager is used, so brightness control can't depend on KWin. Xaver proposed that we use the existing backlight helper from powerdevil and use a new wayland protocol for communication between kwin and powerdevil as DBus may not be responsive enough.

The architecture could look like this:

graph TB

  Action -- "DBus" --> Kact
  Enum -- "DBus" --> plasma-workspace/applets/brightness
  Kact -- "Yes\n\nWayland Protocol" --> HDR
  Kact -- "No (X11)" --> X11
  HDR -- "Wayland Protocol" --> Help
  subgraph plasma-workspace/applets/brightness
  Action[User Action]
  end
  
  subgraph powerdevil/daemon
  Kact{"Kwin present?"}
  Enum("Screen Enumeration (backlighthelper_linux.cpp)")
  X11("Existing code")
  Help("Backlight Helper")
  end

  subgraph Kwin
  HDR("HDR etc. handling")

end

Benefits

  • Less code changes required
  • Support for X11 retained

Drawbacks

  • Complicated architecture
  • Possible performance penalty

Wayland + X11 (3)

Alternatively, KWin can provide the DBus interface for the brightness applet on wayland and powerdevil can continue to provide it on X11.

graph TB

  Action -- "DBus" --> Chk
  Chk -- "Wayland" --> Kwin
  Chk -- "X11" --> Control
  Kwin --> ncnt
  Enum -- "DBus (X11)" --> plasma-workspace/applets/brightness
  Enum2 -- "DBus (Wayland)" --> plasma-workspace/applets/brightness
  subgraph plasma-workspace/applets/brightness
  Action[User Action]
  Chk{"Session Type"}
  end

  subgraph kwin
  Kwin("DBus API provider")
  ncnt("HDR handling + backlight control")
  Enum2("Screen Enumeration")
  end

  subgraph powerdevil
  Control("Existing control")
  Enum("Screen Enumeration")

end

Benefits

  • Basically (1) with X11 support retained
  • Kwin code can be improved independently of powerdevil and X11
  • Later if X11 support is dropped, it would become the same as (1)

Drawbacks

  • Slight code duplication
  • More code changes required

Please share your insights and mention if I missed/misunderstood anything.

Edited by Jakob Petsovits