Default PINModes


I just received my BBB, and I’m trying to understand how everything works. I read the user guide, and saw the extension headers tables detailing pin use. These tables refers to modes (from mode 0 to mode 7). From what I understood, the pin modes are set by the device tree files (under /lib/firmware), and can’t be set at runtime without compiling a new dst file.

So my question: how can I know in which mode is configured each pin ? For example, is the pin P9.21 configured as I2C or as GPIO ?

Thank you,