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wdenk27b207f2003-07-24 23:38:38 +00001Design Notes on Exporting U-Boot Functions to Standalone Applications:
2======================================================================
3
wdenk77846742003-07-26 08:08:08 +000041. 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:
wdenk27b207f2003-07-24 23:38:38 +000012
wdenk77846742003-07-26 08:08:08 +000013 DECLARE_GLOBAL_DATA_PTR;
wdenk27b207f2003-07-24 23:38:38 +000014
wdenk77846742003-07-26 08:08:08 +000015 gd->jt[XF_malloc] = my_malloc;
16 gd->jt[XF_free] = my_free;
wdenk27b207f2003-07-24 23:38:38 +000017
wdenk77846742003-07-26 08:08:08 +000018 Note that the pointers to the functions all have 'void *' type and
19 thus the compiler cannot perform type checks on these assignments.
wdenk27b207f2003-07-24 23:38:38 +000020
wdenk77846742003-07-26 08:08:08 +0000212. The pointer to the jump table is passed to the application in a
Thomas Chou0df01fd2010-05-21 11:08:03 +080022 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.
wdenk27b207f2003-07-24 23:38:38 +000028
wdenk77846742003-07-26 08:08:08 +000029 The application can access the 'global_data' structure in the same
30 way as U-Boot does:
wdenk27b207f2003-07-24 23:38:38 +000031
wdenk77846742003-07-26 08:08:08 +000032 DECLARE_GLOBAL_DATA_PTR;
wdenk27b207f2003-07-24 23:38:38 +000033
wdenk77846742003-07-26 08:08:08 +000034 printf("U-Boot relocation offset: %x\n", gd->reloc_off);
wdenk27b207f2003-07-24 23:38:38 +000035
wdenk77846742003-07-26 08:08:08 +0000363. 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:
wdenk27b207f2003-07-24 23:38:38 +000042
Wolfgang Denk54841ab2010-06-28 22:00:46 +020043 int my_app (int argc, char * const argv[])
wdenk77846742003-07-26 08:08:08 +000044 {
45 app_startup (argv);
46 if (get_version () != XF_VERSION)
47 return 1;
48 }
wdenk27b207f2003-07-24 23:38:38 +000049
wdenk77846742003-07-26 08:08:08 +0000504. The default load and start addresses of the applications are as
51 follows:
wdenk27b207f2003-07-24 23:38:38 +000052
Mike Frysinger4c58eb52008-02-04 19:26:54 -050053 Load address Start address
54 x86 0x00040000 0x00040000
55 PowerPC 0x00040000 0x00040004
56 ARM 0x0c100000 0x0c100000
57 MIPS 0x80200000 0x80200000
58 Blackfin 0x00001000 0x00001000
Macpaul Linafc1ce82011-10-19 20:41:11 +000059 NDS32 0x00300000 0x00300000
Thomas Chou0df01fd2010-05-21 11:08:03 +080060 Nios II 0x02000000 0x02000000
wdenk27b207f2003-07-24 23:38:38 +000061
wdenk77846742003-07-26 08:08:08 +000062 For example, the "hello world" application may be loaded and
63 executed on a PowerPC board with the following commands:
wdenk27b207f2003-07-24 23:38:38 +000064
wdenk77846742003-07-26 08:08:08 +000065 => tftp 0x40000 hello_world.bin
wdenk27b207f2003-07-24 23:38:38 +000066 => go 0x40004
67
wdenk77846742003-07-26 08:08:08 +0000685. To export some additional function foobar(), the following steps
69 should be undertaken:
wdenk27b207f2003-07-24 23:38:38 +000070
wdenk77846742003-07-26 08:08:08 +000071 - Append the following line at the end of the include/_exports.h
72 file:
wdenk27b207f2003-07-24 23:38:38 +000073
wdenk77846742003-07-26 08:08:08 +000074 EXPORT_FUNC(foobar)
wdenk27b207f2003-07-24 23:38:38 +000075
wdenk77846742003-07-26 08:08:08 +000076 - Add the prototype for this function to the include/exports.h
77 file:
wdenk27b207f2003-07-24 23:38:38 +000078
wdenk77846742003-07-26 08:08:08 +000079 void foobar(void);
wdenk27b207f2003-07-24 23:38:38 +000080
wdenk77846742003-07-26 08:08:08 +000081 - Add the initialization of the jump table slot wherever
82 appropriate (most likely, to the jumptable_init() function):
wdenk27b207f2003-07-24 23:38:38 +000083
wdenk77846742003-07-26 08:08:08 +000084 gd->jt[XF_foobar] = foobar;
wdenk27b207f2003-07-24 23:38:38 +000085
wdenk77846742003-07-26 08:08:08 +000086 - Increase the XF_VERSION value by one in the include/exports.h
87 file
wdenk27b207f2003-07-24 23:38:38 +000088
wdenk77846742003-07-26 08:08:08 +0000896. The code for exporting the U-Boot functions to applications is
90 mostly machine-independent. The only places written in assembly
91 language are stub functions that perform the jump through the jump
92 table. That said, to port this code to a new architecture, the
93 only thing to be provided is the code in the examples/stubs.c
94 file. If this architecture, however, uses some uncommon method of
95 passing the 'global_data' pointer (like x86 does), one should add
96 the respective code to the app_startup() function in that file.
wdenk27b207f2003-07-24 23:38:38 +000097
wdenk77846742003-07-26 08:08:08 +000098 Note that these functions may only use call-clobbered registers;
99 those registers that are used to pass the function's arguments,
100 the stack contents and the return address should be left intact.