Add support for the WMI methods used to turn off and adjust the
brightness of the secondary "screenpad" device found on some high-end
ASUS laptops like the GX650P series and others.

Changelog:
- V6
  - Move bulk of cover letter in to commit message
  - Add define for ASUS_SCREENPAD_BRIGHT_DEFAULT to be used on boot
  - Remove the use of max() in update_screenpad_bl_status()
  - Previous: https://lore.kernel.org/all/20230827232358.80512-1-luke@ljones.dev/
- V5
  - Add minimum brightness to prevent screen from becoming invisible
  - Previous: https://lore.kernel.org/all/20230630041743.911303-1-luke@ljones.dev/
- V4
  - Fix a null dereference that happened if the display was powered off and dev struct uninitialised yet
  - Previous: https://lore.kernel.org/all/20230620030033.55033-1-luke@ljones.dev/
- V3
  - Refactor error handling in all functions
- V2
  - Complete refactor to use as a backlight device

Luke D. Jones (1):
  platform/x86: asus-wmi: add support for ASUS screenpad

 drivers/platform/x86/asus-wmi.c            | 133 +++++++++++++++++++++
 drivers/platform/x86/asus-wmi.h            |   1 +
 include/linux/platform_data/x86/asus-wmi.h |   4 +
 3 files changed, 138 insertions(+)

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2.41.0