Cannot disable gamma corection for LEDs (RPi5 ws2812-pio-rp1 module)

[awawa-dev]

Describe the bug

Hi
Kernel module for Raspberry Pi 5 PIO ws2812-pio-rp1 module that allows control over ws2812 sk6812 LED strips:

linux/drivers/misc/ws2812-pio-rp1.c

Line 154 in 2e376fd

p[0] = ws2812_apply_gamma(state->brightness, w);

Gamma correction is hard-coded. We cannot disable it with any settings. So instead of rendering 256 colors per channel we are able to render max 100-120 due to the gamma compression.

Steps to reproduce the behaviour

awawa-dev/HyperHDR#1317

Device (s)

Raspberry Pi 5

System

System Information

Raspberry Pi 5 Model B Rev 1.0
PRETTY_NAME="Debian GNU/Linux 13 (trixie)"
NAME="Debian GNU/Linux"
VERSION_ID="13"
VERSION="13 (trixie)"

Linux rpi5 6.12.47+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.12.47-1+rpt1 (2025-09-16) aarch64 GNU/Linux

Logs

No response

Additional context

No response

[pelwell]

That's a fair point. It would be easy to use a magic brightness value - 0, say - to disable the gamma curve. Would that work for you?

[pelwell]

#7110 is an untested implementation of my idea above. To bypass the gamma curve, add the parameter brightness=0 to the dtoverlay line, e.g.:

dtoverlay=ws2812-pio,gpio=18,num_leds=30,rgbw,brightness=0

By the time you read this you should be able to install a trial kernel using sudo rpi-update pulls/7110. Be careful to back up any important data first, and be extra cautious if you rely on auto_initramfs=1 in order to boot (try booting with it set to 0 before installing the new kernel).

[awawa-dev]

Hi Phil,
Thank you so much for the quick response. It works great now, I'm using a magic byte to set the brightness to 0 to disable gamma. Updated measurements made using a logic analyzer: awawa-dev/HyperHDR#1317

[pelwell]

This issue has now been resolved.