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 and Blackfin architectures 23 use a dedicated register to hold the pointer to the 'global_data' 24 structure: r29 on PowerPC, r8 on ARM, k0 on MIPS, and P5 on Blackfin. 25 The x86 architecture does not use such a register; instead, the pointer 26 to 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); 34 353. 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 } 48 494. 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 Blackfin 0x00001000 0x00001000 58 59 For example, the "hello world" application may be loaded and 60 executed on a PowerPC board with the following commands: 61 62 => tftp 0x40000 hello_world.bin 63 => go 0x40004 64 655. To export some additional function foobar(), the following steps 66 should be undertaken: 67 68 - Append the following line at the end of the include/_exports.h 69 file: 70 71 EXPORT_FUNC(foobar) 72 73 - Add the prototype for this function to the include/exports.h 74 file: 75 76 void foobar(void); 77 78 - Add the initialization of the jump table slot wherever 79 appropriate (most likely, to the jumptable_init() function): 80 81 gd->jt[XF_foobar] = foobar; 82 83 - Increase the XF_VERSION value by one in the include/exports.h 84 file 85 866. The code for exporting the U-Boot functions to applications is 87 mostly machine-independent. The only places written in assembly 88 language are stub functions that perform the jump through the jump 89 table. That said, to port this code to a new architecture, the 90 only thing to be provided is the code in the examples/stubs.c 91 file. If this architecture, however, uses some uncommon method of 92 passing the 'global_data' pointer (like x86 does), one should add 93 the respective code to the app_startup() function in that file. 94 95 Note that these functions may only use call-clobbered registers; 96 those registers that are used to pass the function's arguments, 97 the stack contents and the return address should be left intact. 98