(In reply to comment #44)

> 
> Good news, the radeon card is recognised with an EFI stub boot only !
> 
> But the screens stay black !! (lcd panel and hdmi) ..., X is started because
> I heard lightdm ubuntu sound, but all are black, no X and no console frame
> buffer.

It looks like you have a hybrid system with both an integrated GPU and a
discrete GPU.  You'll need to switch the mux to switch between the two GPUs
(the integrated is probably selected by default).  Apple has their own
proprietary mux and I'm not sure how well it's supported in Linux.