I noted that the actual brightness of my laptop monitor is different on the same value(s) for backlight from the same range of available values when running Linux with kernel 5.4 vs. 3.13 (i915 driver). What might have changed to result in that? I like the 3.13 one better.
TL;DR
I noted backlight works differently on my laptop now on 5.4 kernel than on 3.13, range of values to write to /sys/class/backlight/intel_backlight/brightness is the same, but perceived brightness at 1 is much lower on 3.13 whereas max seems same.
I've found out most probably backlight on my laptop is controlled by i915 driver and source code in https://github.com/torvalds/linux/tree/master/drivers/gpu/drm/i915 (it is a mirror per Why does the Linux kernel repository have only one branch?, but on kernel.org I do not know how to open a tree of code to provide a web link to the driver).
How to find code that process input to /sys/class/backlight/intel_backlight/brightness? Hints on what to do next are also welcomed.