1Design Notes on Exporting U-Boot Functions to Standalone Applications:
2======================================================================
3
41. The functions are exported by U-Boot via a jump table. The jump
5   table is allocated and initialized in the jumptable_init() routine
6   (common/exports.c). Other routines may also modify the jump table,
7   however. The jump table can be accessed as the 'jt' field of the
8   'global_data' structure. The struct members for the jump table are
9   defined in the <include/exports.h> header. E.g., to substitute the
10   malloc() and free() functions that will be available to standalone
11   applications, one should do the following:
12
13	DECLARE_GLOBAL_DATA_PTR;
14
15	gd->jt->malloc	= my_malloc;
16	gd->jt->free = my_free;
17
18   Note that the pointers to the functions are real function pointers
19   so the compiler can perform type checks on these assignments.
20
212. The pointer to the jump table is passed to the application in a
22   machine-dependent way. PowerPC, ARM, MIPS, Blackfin and Nios II
23   architectures use a dedicated register to hold the pointer to the
24   'global_data' structure: r2 on PowerPC, r9 on ARM, k0 on MIPS,
25   P3 on Blackfin and gp on Nios II. The x86 architecture does not
26   use such a register; instead, the pointer to the 'global_data'
27   structure is passed as 'argv[-1]' pointer.
28
29   The application can access the 'global_data' structure in the same
30   way as U-Boot does:
31
32	DECLARE_GLOBAL_DATA_PTR;
33
34	printf("U-Boot relocation offset: %x\n", gd->reloc_off);
35
363. The application should call the app_startup() function before any
37   call to the exported functions. Also, implementor of the
38   application may want to check the version of the ABI provided by
39   U-Boot. To facilitate this, a get_version() function is exported
40   that returns the ABI version of the running U-Boot. I.e., a
41   typical application startup may look like this:
42
43	int my_app (int argc, char *const argv[])
44	{
45		app_startup (argv);
46		if (get_version () != XF_VERSION)
47			return 1;
48	}
49
504. The default load and start addresses of the applications are as
51   follows:
52
53			Load address	Start address
54	x86		0x00040000	0x00040000
55	PowerPC		0x00040000	0x00040004
56	ARM		0x0c100000	0x0c100000
57	MIPS		0x80200000	0x80200000
58	Blackfin	0x00001000	0x00001000
59	NDS32		0x00300000	0x00300000
60	Nios II		0x02000000	0x02000000
61	RISC-V		0x00600000	0x00600000
62
63   For example, the "hello world" application may be loaded and
64   executed on a PowerPC board with the following commands:
65
66   => tftp 0x40000 hello_world.bin
67   => go 0x40004
68
695. To export some additional function long foobar(int i,char c), the following steps
70   should be undertaken:
71
72   - Append the following line at the end of the include/_exports.h
73     file:
74
75	EXPORT_FUNC(foobar, long, foobar, int, char)
76
77	Parameters to EXPORT_FUNC:
78	 - the first parameter is the function that is exported (default implementation)
79	 - the second parameter is the return value type
80	 - the third  parameter is the name of the member in struct jt_funcs
81	   this is also the name that the standalone application will used.
82	   the rest of the parameters are the function arguments
83
84   - Add the prototype for this function to the include/exports.h
85     file:
86
87	long foobar(int i, char c);
88
89	Initialization with the default implementation is done in jumptable_init()
90
91	You can override the default implementation using:
92
93	gd->jt->foobar = another_foobar;
94
95	The signature of another_foobar must then match the declaration of foobar.
96
97   - Increase the XF_VERSION value by one in the include/exports.h
98     file
99
100   - If you want to export a function which depends on a CONFIG_XXX
101	use 2 lines like this:
102	#ifdef CONFIG_FOOBAR
103		EXPORT_FUNC(foobar, long, foobar, int, char)
104	#else
105		EXPORT_FUNC(dummy, void, foobar, void)
106	#endif
107
108
1096. The code for exporting the U-Boot functions to applications is
110   mostly machine-independent. The only places written in assembly
111   language are stub functions that perform the jump through the jump
112   table. That said, to port this code to a new architecture, the
113   only thing to be provided is the code in the examples/stubs.c
114   file. If this architecture, however, uses some uncommon method of
115   passing the 'global_data' pointer (like x86 does), one should add
116   the respective code to the app_startup() function in that file.
117
118   Note that these functions may only use call-clobbered registers;
119   those registers that are used to pass the function's arguments,
120   the stack contents and the return address should be left intact.
121