No, the problem is that the optical sensor (hardware) looks like is broke. The back cover is fine, I even tried with other back cover that I have spare here and nothing happened. Oh no, that was my main fear... so I should rewrite some of the code, and then I need to recompile and learn how to synchronize with the kernel... oh my... I'll loose all the rest of my leisure time on this... OR I could try to replace the optical sensor for an equivalent one. Thank you for the answer, sorry if I'm not happy for it