Systemd is not used and SysV is used.This is where Yocto can be totally different from Raspberry Pi OS.
The systemd-backlight service saves the backlight brightness on shutdown and restores it on restart. I've never noticed an issue of the backlight not coming on, but it may be due to that always running.
The backlight is attached to the relevant panel driver via device tree, so drm_panel_[enable|disable] should automatically turn it on/off. The brightness property defaults to max as well, although REG_PWM is written as 0 initially.
Is it possible that the time when the entry /sys/class/backlight/10-0045/brightness is created earlier than the time of backlight probing(), or specially earlier than the time when the setting is actually written to the device, due to the asynchronous probe type or any other reasons?
Here is the dmesg regarding backlight probing when udev rule is used:
root@raspberrypi4-64:~# dmesg | grep -i probe | grep 10-0045
[ 3.651208] platform 100000001.panel_disp: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.651401] platform 100000002.reg_bridge: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.741974] platform 100000001.panel_disp: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.742138] platform 100000002.reg_bridge: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.775031] platform 100000001.panel_disp: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.786701] platform 100000002.reg_bridge: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.876806] i2c 10-0038: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.918241] bus: 'i2c': __driver_probe_device: matched device 10-0045 with driver rpi_touchscreen_attiny
[ 3.947938] bus: 'i2c': really_probe: probing driver rpi_touchscreen_attiny with device 10-0045
[ 3.974691] platform 100000001.panel_disp: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.974847] platform 100000002.reg_bridge: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 3.974872] i2c 10-0038: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 4.049329] platform 100000001.panel_disp: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 4.049490] platform 100000002.reg_bridge: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 4.049521] i2c 10-0038: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 4.072601] platform 100000001.panel_disp: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 4.072740] platform 100000002.reg_bridge: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 4.072796] i2c 10-0038: error -EPROBE_DEFER: supplier 10-0045 not ready
[ 4.131147] bus: 'i2c': really_probe: bound device 10-0045 to driver rpi_touchscreen_attiny
root@raspberrypi4-64:~#
Statistics: Posted by crane — Fri Jul 11, 2025 2:58 pm