Where is the source code that implements the driver for /dev/ttyS0 serial port in Red Hat?

I am trying to debug a problem with a device attached to /dev/ttyS0 on a workstation running RHEL 5. Using my relatively basic Linux knowledge, I can not seem to figure out where to look for the source code for the serial port driver. Where is it and what is it called?

From serverfault Joshua Lee

• If you want the RHEL5 package, you can download the kernel source RPM here. If you want the kernel source from the primary distribution, you can get it from kernel.org.

  Depending on the particular driver or part of the kernel you want to look at, it may be in the linux/drivers/serial directory.

  From Warner