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