OK, the right grub option was "nomodeset". With that option, KMS is disabled (so Ctrl+Alt+Fx takes a "long" time), and I can normally set the brightness with the Fn Up/Down keys.
OK, the right grub option was "nomodeset". With that option, KMS is disabled (so Ctrl+Alt+Fx takes a "long" time), and I can normally set the brightness with the Fn Up/Down keys.