I was using the nvidia proprietary driver. After removing this driver and instead using the open source driver, the brightness control works. I would still want to use the nvidia driver as the open source driver does not give me the full resolution. Seems my case is similar to gidantribal's. Hope to see a solution soon!
An update.
I was using the nvidia proprietary driver. After removing this driver and instead using the open source driver, the brightness control works. I would still want to use the nvidia driver as the open source driver does not give me the full resolution. Seems my case is similar to gidantribal's. Hope to see a solution soon!