fbcon: Document what I learned about fbcon locking

#  define DPRINTK(fmt, args...)

#endif

/*

 * FIXME: Locking

 *

 * - fbcon state itself is protected by the console_lock, and the code does a

 *   pretty good job at making sure that lock is held everywhere it's needed.

 *

 * - access to the registered_fb array is entirely unprotected. This should use

 *   proper object lifetime handling, i.e. get/put_fb_info. This also means

 *   switching from indices to proper pointers for fb_info everywhere.

 *

 * - fbcon doesn't bother with fb_lock/unlock at all. This is buggy, since it

 *   means concurrent access to the same fbdev from both fbcon and userspace

 *   will blow up. To fix this all fbcon calls from fbmem.c need to be moved out

 *   of fb_lock/unlock protected sections, since otherwise we'll recurse and

 *   deadlock eventually. Aside: Due to these deadlock issues the fbdev code in

 *   fbmem.c cannot use locking asserts, and there's lots of callers which get

 *   the rules wrong, e.g. fbsysfs.c entirely missed fb_lock/unlock calls too.

 */

enum {

	FBCON_LOGO_CANSHOW	= -1,	/* the logo can be shown */

	FBCON_LOGO_DRAW		= -2,	/* draw the logo to a console */