The uapi folder is supposed to contain the user space API of the kernel. Then upon kernel installation, the uapi include files become the top level /usr/include/linux/ files. (I’m not entirely clear on what exceptions remain.)
The other headers in theory are then private to the kernel. This allow clean separation of the user-visible and kernel-only structures which previously were intermingled in a single header file.
The best discussion I have seen of this is located at a Linux Weekly News article that predates the patch landing.
The UAPI patch itself landed with kernel 3.7. Linus’s quick and dirty summary is:
the “uapi” include file cleanups. The idea is that the stuff
exported to user space should now be found under include/uapi and
arch/$(ARCH)/include/uapi.Let’s hope it actually works. Because otherwise this was just a
totally pointless pain in the *ss. And regardless, I’m definitely done
with these kinds of “let’s do massive cleanup of the include files”
forever.