I am currently playing Portal 2 on my brand new Radeon R9 390, and I have found that there is a graphic bug when displaying the rock textures often found outside. I have made a screenshot, and of course I have validated all of my game files through Steam. So I guess there is a driver problem in the latest Crimson driver that prevents the game from displaying correctly. Please have a look at the screenshot I made, and especially look at the rock texture:
Is there any fix available?