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Some two years ago, I was able to set a very dim backlight brightness by writing a non-integer value to /sys/class/backlight/intel_backlight/brightness.

$ echo 0.3 > /sys/class/backlight/intel_backlight/brightness

But now, it seems that there is some sanity check... so, the system complains:

bash: echo: write error: Invalid argument

Is there anyway I can bypass such sanity check? Is there a way to pass values directly to the driver? I believe the relevant driver is i915.

Linux debiel 5.4.0-4-amd64 #1 SMP Debian 5.4.19-1 (2020-02-13) x86_64 GNU/Linux

Please, let me know if I should have given you any useful information. I do not really know how to properly report the problem.

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Sounds like an implementation detail of your specific hardware driver. Have you used the exact same hardware when setting float dim values resulted in actually less lit display? Or do you now have simply a less finely configurable backlight, maybe? Probably, you could go to an intel support forum and ask there for the backlight value stepping API. Would be interesting, what their officials tell then. At least intel releases their own linux hardware drivers, so this is officially specified APIs. As I like Arch Linux Wiki for such information, I post a link to their API description here:

https://wiki.archlinux.org/index.php/Backlight#Backlight_PWM_modulation_frequency_(Intel_i915_only)

This Link explains i915 is using PWM to adjust light more accurately. Maybe PWM is disabled in your kernel since it typucally caused flickering on this hardware.

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