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When I start my Linux system (Debian/stable), I see that the last line of dmesg states:

[   44.043657] rfkill: input handler disabled

What does this mean?

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rfkill is a Linux kernel subsystem providing access to hardware and software toggle switches that enable or disable radio transmitters, such as wireless network cards and Bluetooth dongles. The Linux kernel contains an apparently deprecated functionality called rfkill-input that toggles an rfkill switch from inside the kernel in response to pressing a dedicated key on the keyboard (usually labelled with a radio transmitter icon). Userspace software that wants to take over handling such keypresses can disable this built-in functionality by opening /dev/rfkill and invoking ioctl with the command RFKILL_IOC_NOINPUT. When this happens, the kernel emits the message mentioned in the question.

In other words, it’s harmless; it (probably) only means that some userspace software (for example your desktop environment) took over handling of the ‘disable radio’ key. Running fuser /dev/rfkill may help you discover which particular process it is.

You should expect that the message will disappear altogether in a future kernel version that removes rfkill-input, with the kernel expecting that userspace shall always be responsible for responding to such keypresses.

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