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wdenk874ac262003-07-24 23:38:38 +00001Design Notes on Exporting U-Boot Functions to Standalone Applications:
2======================================================================
3
wdenkb8463562003-07-26 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:
wdenk874ac262003-07-24 23:38:38 +000012
wdenkb8463562003-07-26 08:08:08 +000013 DECLARE_GLOBAL_DATA_PTR;
wdenk874ac262003-07-24 23:38:38 +000014
wdenkb8463562003-07-26 08:08:08 +000015 gd->jt[XF_malloc] = my_malloc;
16 gd->jt[XF_free] = my_free;
wdenk874ac262003-07-24 23:38:38 +000017
wdenkb8463562003-07-26 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.
wdenk874ac262003-07-24 23:38:38 +000020
wdenkb8463562003-07-26 08:08:08 +0000212. The pointer to the jump table is passed to the application in a
Thomas Chou8fa38582010-05-21 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.
wdenk874ac262003-07-24 23:38:38 +000028
wdenkb8463562003-07-26 08:08:08 +000029 The application can access the 'global_data' structure in the same
30 way as U-Boot does:
wdenk874ac262003-07-24 23:38:38 +000031
wdenkb8463562003-07-26 08:08:08 +000032 DECLARE_GLOBAL_DATA_PTR;
wdenk874ac262003-07-24 23:38:38 +000033
wdenkb8463562003-07-26 08:08:08 +000034 printf("U-Boot relocation offset: %x\n", gd->reloc_off);
wdenk874ac262003-07-24 23:38:38 +000035
wdenkb8463562003-07-26 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:
wdenk874ac262003-07-24 23:38:38 +000042
wdenkb8463562003-07-26 08:08:08 +000043 int my_app (int argc, char *argv[])
44 {
45 app_startup (argv);
46 if (get_version () != XF_VERSION)
47 return 1;
48 }
wdenk874ac262003-07-24 23:38:38 +000049
wdenkb8463562003-07-26 08:08:08 +0000504. The default load and start addresses of the applications are as
51 follows:
wdenk874ac262003-07-24 23:38:38 +000052
Mike Frysinger60f09302008-02-04 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
Thomas Chou8fa38582010-05-21 11:08:03 +080059 Nios II 0x02000000 0x02000000
wdenk874ac262003-07-24 23:38:38 +000060
wdenkb8463562003-07-26 08:08:08 +000061 For example, the "hello world" application may be loaded and
62 executed on a PowerPC board with the following commands:
wdenk874ac262003-07-24 23:38:38 +000063
wdenkb8463562003-07-26 08:08:08 +000064 => tftp 0x40000 hello_world.bin
wdenk874ac262003-07-24 23:38:38 +000065 => go 0x40004
66
wdenkb8463562003-07-26 08:08:08 +0000675. To export some additional function foobar(), the following steps
68 should be undertaken:
wdenk874ac262003-07-24 23:38:38 +000069
wdenkb8463562003-07-26 08:08:08 +000070 - Append the following line at the end of the include/_exports.h
71 file:
wdenk874ac262003-07-24 23:38:38 +000072
wdenkb8463562003-07-26 08:08:08 +000073 EXPORT_FUNC(foobar)
wdenk874ac262003-07-24 23:38:38 +000074
wdenkb8463562003-07-26 08:08:08 +000075 - Add the prototype for this function to the include/exports.h
76 file:
wdenk874ac262003-07-24 23:38:38 +000077
wdenkb8463562003-07-26 08:08:08 +000078 void foobar(void);
wdenk874ac262003-07-24 23:38:38 +000079
wdenkb8463562003-07-26 08:08:08 +000080 - Add the initialization of the jump table slot wherever
81 appropriate (most likely, to the jumptable_init() function):
wdenk874ac262003-07-24 23:38:38 +000082
wdenkb8463562003-07-26 08:08:08 +000083 gd->jt[XF_foobar] = foobar;
wdenk874ac262003-07-24 23:38:38 +000084
wdenkb8463562003-07-26 08:08:08 +000085 - Increase the XF_VERSION value by one in the include/exports.h
86 file
wdenk874ac262003-07-24 23:38:38 +000087
wdenkb8463562003-07-26 08:08:08 +0000886. 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.
wdenk874ac262003-07-24 23:38:38 +000096
wdenkb8463562003-07-26 08:08:08 +000097 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.