Design Notes on Exporting U-Boot Functions to Standalone Applications:
  ======================================================================
  
  1. The functions are exported by U-Boot via a jump table. The jump
     table is allocated and initialized in the jumptable_init() routine
     (common/exports.c). Other routines may also modify the jump table,
     however. The jump table can be accessed as the 'jt' field of the
     'global_data' structure. The struct members for the jump table are
     defined in the <include/exports.h> header. E.g., to substitute the
     malloc() and free() functions that will be available to standalone
     applications, one should do the following:
  
  	DECLARE_GLOBAL_DATA_PTR;
  
  	gd->jt->malloc	= my_malloc;
  	gd->jt->free = my_free;
  
     Note that the pointers to the functions are real function pointers
     so the compiler can perform type checks on these assignments.
  
  2. The pointer to the jump table is passed to the application in a
     machine-dependent way. PowerPC, ARM, MIPS, Blackfin and Nios II
     architectures use a dedicated register to hold the pointer to the
     'global_data' structure: r2 on PowerPC, r9 on ARM, k0 on MIPS,
     P3 on Blackfin and gp on Nios II. The x86 architecture does not
     use such a register; instead, the pointer to the 'global_data'
     structure is passed as 'argv[-1]' pointer.
  
     The application can access the 'global_data' structure in the same
     way as U-Boot does:
  
  	DECLARE_GLOBAL_DATA_PTR;
  
  	printf("U-Boot relocation offset: %x
  ", gd->reloc_off);
  
  3. The application should call the app_startup() function before any
     call to the exported functions. Also, implementor of the
     application may want to check the version of the ABI provided by
     U-Boot. To facilitate this, a get_version() function is exported
     that returns the ABI version of the running U-Boot. I.e., a
     typical application startup may look like this:
  
  	int my_app (int argc, char * const argv[])
  	{
  		app_startup (argv);
  		if (get_version () != XF_VERSION)
  			return 1;
  	}
  
  4. The default load and start addresses of the applications are as
     follows:
  
  			Load address	Start address
  	x86		0x00040000	0x00040000
  	PowerPC		0x00040000	0x00040004
  	ARM		0x0c100000	0x0c100000
  	MIPS		0x80200000	0x80200000
  	Blackfin	0x00001000	0x00001000
  	NDS32		0x00300000	0x00300000
  	Nios II		0x02000000	0x02000000
  	RISC-V		0x00600000	0x00600000
  
     For example, the "hello world" application may be loaded and
     executed on a PowerPC board with the following commands:
  
     => tftp 0x40000 hello_world.bin
     => go 0x40004
  
  5. To export some additional function long foobar(int i,char c), the following steps
     should be undertaken:
  
     - Append the following line at the end of the include/_exports.h
       file:
  
  	EXPORT_FUNC(foobar, long, foobar, int, char)
  
  	Parameters to EXPORT_FUNC:
  	 - the first parameter is the function that is exported (default implementation)
  	 - the second parameter is the return value type
  	 - the third  parameter is the name of the member in struct jt_funcs
  	   this is also the name that the standalone application will used.
  	   the rest of the parameters are the function arguments
  
     - Add the prototype for this function to the include/exports.h
       file:
  
  	long foobar(int i, char c);
  
  	Initialization with the default implementation is done in jumptable_init()
  
  	You can override the default implementation using:
  
  	gd->jt->foobar = another_foobar;
  
  	The signature of another_foobar must then match the declaration of foobar.
  
     - Increase the XF_VERSION value by one in the include/exports.h
       file
  
     - If you want to export a function which depends on a CONFIG_XXX
  	use 2 lines like this:
  	#ifdef CONFIG_FOOBAR
  		EXPORT_FUNC(foobar, long, foobar, int, char)
  	#else
  		EXPORT_FUNC(dummy, void, foobar, void)
  	#endif
  
  
  6. The code for exporting the U-Boot functions to applications is
     mostly machine-independent. The only places written in assembly
     language are stub functions that perform the jump through the jump
     table. That said, to port this code to a new architecture, the
     only thing to be provided is the code in the examples/stubs.c
     file. If this architecture, however, uses some uncommon method of
     passing the 'global_data' pointer (like x86 does), one should add
     the respective code to the app_startup() function in that file.
  
     Note that these functions may only use call-clobbered registers;
     those registers that are used to pass the function's arguments,
     the stack contents and the return address should be left intact.