[Exim] Exim 4 directive "log_file_path" and "$interface_port…

I setup Exim to listen on the port 25 and 26. 25 is inbound and 26 is outbound. And wanna use two separate log file, so set the log_file_path as:
...
log_file_path = /var/log/exim/$interface_port/%s.log
...

but it always loged into file "/var/log/exim/-1/%s.log".

What happen?