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1 | Design Notes on Exporting U-Boot Functions to Standalone Applications: |
2 | ====================================================================== | |
3 | ||
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4 | 1. 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 | ||
21 | 2. The pointer to the jump table is passed to the application in a | |
22 | machine-dependent way. PowerPC, ARM and MIPS architectures use a | |
23 | dedicated register to hold the pointer to the 'global_data' | |
24 | structure: r29 on PowerPC, r8 on ARM and k0 on MIPS. The x86 | |
25 | architecture does not use such a register; instead, the pointer to | |
26 | the 'global_data' structure is passed as 'argv[-1]' pointer. | |
27 | ||
28 | The application can access the 'global_data' structure in the same | |
29 | way as U-Boot does: | |
30 | ||
31 | DECLARE_GLOBAL_DATA_PTR; | |
32 | ||
33 | printf("U-Boot relocation offset: %x\n", gd->reloc_off); | |
34 | ||
35 | 3. The application should call the app_startup() function before any | |
36 | call to the exported functions. Also, implementor of the | |
37 | application may want to check the version of the ABI provided by | |
38 | U-Boot. To facilitate this, a get_version() function is exported | |
39 | that returns the ABI version of the running U-Boot. I.e., a | |
40 | typical application startup may look like this: | |
41 | ||
42 | int my_app (int argc, char *argv[]) | |
43 | { | |
44 | app_startup (argv); | |
45 | if (get_version () != XF_VERSION) | |
46 | return 1; | |
47 | } | |
48 | ||
49 | 4. The default load and start addresses of the applications are as | |
50 | follows: | |
51 | ||
52 | Load address Start address | |
53 | x86 0x00040000 0x00040000 | |
54 | PowerPC 0x00040000 0x00040004 | |
55 | ARM 0x0c100000 0x0c100000 | |
56 | MIPS 0x80200000 0x80200000 | |
57 | ||
58 | For example, the "hello world" application may be loaded and | |
59 | executed on a PowerPC board with the following commands: | |
60 | ||
61 | => tftp 0x40000 hello_world.bin | |
62 | => go 0x40004 | |
27b207fd | 63 | |
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64 | 5. To export some additional function foobar(), the following steps |
65 | should be undertaken: | |
27b207fd | 66 | |
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67 | - Append the following line at the end of the include/_exports.h |
68 | file: | |
27b207fd | 69 | |
77846748 | 70 | EXPORT_FUNC(foobar) |
27b207fd | 71 | |
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72 | - Add the prototype for this function to the include/exports.h |
73 | file: | |
27b207fd | 74 | |
77846748 | 75 | void foobar(void); |
27b207fd | 76 | |
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77 | - Add the initialization of the jump table slot wherever |
78 | appropriate (most likely, to the jumptable_init() function): | |
27b207fd | 79 | |
77846748 | 80 | gd->jt[XF_foobar] = foobar; |
27b207fd | 81 | |
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82 | - Increase the XF_VERSION value by one in the include/exports.h |
83 | file | |
27b207fd | 84 | |
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85 | 6. The code for exporting the U-Boot functions to applications is |
86 | mostly machine-independent. The only places written in assembly | |
87 | language are stub functions that perform the jump through the jump | |
88 | table. That said, to port this code to a new architecture, the | |
89 | only thing to be provided is the code in the examples/stubs.c | |
90 | file. If this architecture, however, uses some uncommon method of | |
91 | passing the 'global_data' pointer (like x86 does), one should add | |
92 | the respective code to the app_startup() function in that file. | |
27b207fd | 93 | |
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94 | Note that these functions may only use call-clobbered registers; |
95 | those registers that are used to pass the function's arguments, | |
96 | the stack contents and the return address should be left intact. |