"Mainland Asia" includes "the lower basin of China's Yangtze River" as well as "Korea".  But being specific is fine too.  I just found a citation for a more comprehensive DNA study by Hammer below, rather than our generarizations and speculation so far. 

 Citation for "Yayoi culture was brought to Japan by migrants from Korea, who in turn trace their roots to southeast Asia/south China." 

 2005 DNA study by Hammer
 Describes the Yayoi migration from Korea based on the O-SRY(465) genes and other genes with close lineage (haplogroups O-M122 and O-M95).
Reiterates that "the entire O haplogroup has been proposed to have a Southeast Asian origin."  (Their definition of Southeast Asia includes southern China).  Then hypothesizes that "the dispersals of Neolithic farmers from Southeast Asia also brought haplogroup O lineages to Korea and eventually to Japan."
 In the concluding paragraph, it states "we propose that the Yayoi Y chromosomes descend from prehistoric farmers that had their origins in southeastern Asia, perhaps going back to the origin of agriculture in this region."
 Hammer's DNA study is based on a "global sample consisted of > 2,500 males from 39 Asian populations, including six populations sampled from across the Japanese archipelago."