"Who knew what and when" is a red herring. It means nothing unless it contradicts sworn testimony, or if the "what" is illegal. If, in a discussion with Obama, Holder contradicts his sworn testimony then you are out of luck -- you'll never get that piece of paper because of executive privilege (regardless of why it was invoked). Otherwise, "knowing" about F and F is not a crime. Indeed, I have not seen it argued that F and F itself was a crime (I may have seen some wild claims, but no real argument).