Where is in the firmware the pin for CEC defined?
It is not that easy as you might think. There is no CEC pin that can be accessed by the "firmware". CEC pins typically ends at a dedicated CEC chip that is in or near the CPU. There are many different kinds of CEC chips. For everyone you need a kernel driver. The userspace communicates with this kernel driver via an adapter in the LibCEC. Without any knowledge of the used CEC chip it is impossible to write a kernel driver.
The schematics you have linked, does only show that the CEC line goes to a level shifter, but the CEC output of this chip ends in the nowhere. So there is even no proof that CEC would work with this device.
Gerald
[hr]
Okay, I understand now that you want to root the CEC pin to a GPIO pin. AFAIK there is no kernel driver available that is able to do CEC communication directly with the CEC line. The only peace of software I know of, that is doing this, is a firmware for an atmega microcontroller. Maybe someone wants to try to port this to a kernel driver, but this will be extremely difficult. In contrast to the Atom CPU running distributions like LE, the atmega gets not disturbed by interrupts from other hardware. So it will be very difficult to keep the timing constraints of the CEC bus, if e.g. a userspace writes something to the disk, or the user types on the keyboard. To reserve just on core of the Atom for CEC communication seems not to be a good idea
That is a task I surely will not do, but if somebody else steps in here and is doing a kernel driver, and someone provides me a free sample of this hardware, I will do the adapter for the libcec.
Gerald