5.4.3.1.2 Detailing for local ductility
a) at the compression zone reinforcement of not less than half of the reinforcement provided at the tension zone is placed, in addition to any compression reinforcement needed for the ULS verification of the beam in the seismic design situation.
I understand that, the compression reinforcement shall not be less than half of the reinforcement provided at the tension zone ( span reinforcement ) . That is, calculate the compression reinforcement acc. to ULS loading ,(including the stress reversal for seismic case ) and provide compression reinforcement which satisfy AT LEAST HALF OF THE TENSION REINFORCEMENT PROVIDED AT SPAN..
This is an old rule to provide at least half of the main bars specified at span should continue at supports.