xref: /openbmc/u-boot/doc/README.standalone (revision 14d0a02a)
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 slot numbers 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[XF_malloc]	= my_malloc;
16	gd->jt[XF_free]		= my_free;
17
18   Note that the pointers to the functions all have 'void *' type and
19   thus the compiler cannot 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, r8 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	Nios II		0x02000000	0x02000000
60
61   For example, the "hello world" application may be loaded and
62   executed on a PowerPC board with the following commands:
63
64   => tftp 0x40000 hello_world.bin
65   => go 0x40004
66
675. To export some additional function foobar(), the following steps
68   should be undertaken:
69
70   - Append the following line at the end of the include/_exports.h
71     file:
72
73	EXPORT_FUNC(foobar)
74
75   - Add the prototype for this function to the include/exports.h
76     file:
77
78	void foobar(void);
79
80   - Add the initialization of the jump table slot wherever
81     appropriate (most likely, to the jumptable_init() function):
82
83	gd->jt[XF_foobar] = foobar;
84
85   - Increase the XF_VERSION value by one in the include/exports.h
86     file
87
886. The code for exporting the U-Boot functions to applications is
89   mostly machine-independent. The only places written in assembly
90   language are stub functions that perform the jump through the jump
91   table. That said, to port this code to a new architecture, the
92   only thing to be provided is the code in the examples/stubs.c
93   file. If this architecture, however, uses some uncommon method of
94   passing the 'global_data' pointer (like x86 does), one should add
95   the respective code to the app_startup() function in that file.
96
97   Note that these functions may only use call-clobbered registers;
98   those registers that are used to pass the function's arguments,
99   the stack contents and the return address should be left intact.
100