Now I'm totally confused, I can't find helpful information from Internet.

I know mesa is a open source implementation of OpenGL, obviously

mesa will afford OpenGL API. DRI is short for Direct Rendering Infrastructure,

I have chosen options like:

Device Drivers --->
  Graphics support --->
  <*>  Direct Rendering Manager --->
  <*>    ATI Radeon
  [*]      Enable modesetting on radeon by default

Does this mean that DRI libraries are built into kernel? If not, who contains DRI?

Also who affords GLX libraries? Mesa or Xorg?