[RFC] Splitting kernel headers and deprecating __KERNEL__

[David Howells <dhowells@redhat.com>]

We've been discussing splitting the kernel headers into userspace API headers
and kernel internal headers and deprecating the __KERNEL__ macro. This will
permit a cleaner interface between the kernel and userspace; and one that's
easier to keep up to date.

What we've come up with is this:

 (1) Create new directories in the linux sources to shadow existing include
     directories:

	NEW DIRECTORY		DIRECTORY SHADOWED
	=============		==================
	include/user/		include/linux/
	include/user-*/		include/asm-*/

     Note that this doesn't take account of the other directories under
     include/, but I don't think they're relevant.

 (2) Take each file from the shadowed directory. If it has any userspace
     relevant stuff, then:

     (a) Transfer this stuff into a file of the same name in the new
	 directory. So, for example, the syscall number list from
	 include/asm-i386/unistd.h will be transferred to
	 include/user-i386/unistd.h.

     (b) Make kernel file #include the user file. So:

		[include/asm-i386/unistd.h]
		...
		#include <user-i386/unistd.h>
		...

     (c) Where a user header file requires something from another header file
	 (such as a type), that file should include a suitable user header file
	 directly:

		[include/user-i386/termio.h]
		...
		#include <user/types.h>
		...

     (d) stdint types should be used where possible.

		[include/user-i386/termios.h]
		struct winsize {
			uint16_t ws_row;
			uint16_t ws_col;
			uint16_t ws_xpixel;
			uint16_t ws_ypixel;
		};

     (e) These header files should be bounded with __USER_XXXXX_H conditionals:

		[include/user-i386/termios.h]
		#ifndef __USER_I386_TERMIOS_H
		#define __USER_I386_TERMIOS_H
		...
		#endif /*  __USER_I386_TERMIOS_H */

 (3) Remove all #if(n)def __KERNEL__ clauses.

 (4) Remove the -D__KERNEL__ from the master kernel Makefile.

 (5) For userspace use (such as for glibc), the appropriate include/user*/
     directories should be selected and installed in /usr/include/ or wherever,
     and symlinks made. For example, on i386 arch boxes, you might find:

	SOURCE			INSTALLED AS
	======================	============
	include/user/		/usr/include/user/
	include/user-i386/	/usr/include/user-i386/
				/usr/include/linux -> user
				/usr/include/asm -> user-i386

 (6) On multi-arch archs (such as ppc64 which can also support ppc), you might
     find:

	SOURCE			INSTALLED AS
	======================	============
	include/user/		/usr/include/user/
	include/user-ppc/	/usr/include/user-ppc/
	include/user-ppc64/	/usr/include/user-ppc64/
				/usr/include/linux -> user
				/usr/include/asm-ppc -> user-ppc
				/usr/include/asm-ppc64 -> user-ppc64

     And then /usr/include/asm/ might contain files that do arch-size dependent
     switching between user-ppc and user-ppc64.


David