[Bug] 0.14.33 breaks backlight levels

[karlshea]

Describe the Bug

I have a higher number of backlight levels (20) so it's more obvious, but even a smaller number shows this behavior: increasing/decreasing the backlight levels jumps the light intensity all over the place (e.g. starting from backlight level 0 -> intensity 0, 1 -> intensity 50 (estimated), 2 -> intensity 10, 3 -> intensity 200, 4 -> intensity 50, 5 -> intensity 190).

0.14.32 works normally. If I check out the first commit (b47bdee ) after that tag it starts to break.

System Information

• Keyboard: Both KBDfans KBD75 rev2 and Treasure type9s2 show this behavior.
• Operating system: macOS
• AVR GCC version:

• ARM GCC version:

• QMK Firmware version:

• Any keyboard related software installed?
  • AutoHotKey
  • Karabiner
  • Other:

Additional Context

Building using latest qmkfm/qmk_cli Docker image.

[tzarc]

Do you know how to do a git bisect?

[karlshea]

I didn't, but now I do, it says e0a5056 is the first bad commit.

Edit: confirmed, if I check out 0.14.33 but replace quantum/backlight/backlight_avr.c with the version prior to that commit, everything works fine.

Edit 2: Since many of the changes in the PR (#7521) seem to be related to backlight breathing, just wanted to clarify that I'm not using breathing, it's just a static level.

[karlshea]

It's something to do with cie_lightness(). If I revert just those changes it works fine too.

[karlshea]

Figured it out, it's losing bits doing division.