A processor core is supported by an upper level cache and a lower level cache that receives, from an interconnect fabric, a write injection request requesting injection of a partial cache line of data into a target cache line identified by a target real address. In response to receipt of the write injection request, a determination is made that the upper level cache is a highest point of coherency for the target real address. In response to the determination, the upper level cache and lower level cache collaborate to transfer the target cache line from the upper level cache to the lower level cache. The lower level cache updates the target cache line by merging the partial cache of data into the target cache line and storing the updated target cache line in the lower level cache.