Need an easy way to have your Arduino talk to your Raspberry PI? Oscar has you covered. The naturally obvious solution is just to run the USB serial from the Arduino to the Raspberry PI’s USB host. Add a quick arduino sketch, some python using pyserial and you’re in business.
If you’ve modded your Raspberry PI’s power supply and have a decent power output on your ports, you can just run the Arduino from the PI.
Should only take you a few minutes to get this up and running. Check it out.