doc/README.standalone

   1 Design Notes on Exporting U-Boot Functions to Standalone Applications:
   2 ======================================================================
   3
   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, MIPS, Blackfin and Nios II
  23    architectures use a dedicated register to hold the pointer to the
  24    'global_data' structure: r2 on PowerPC, r9 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
  36 3. 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
  50 4. 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         NDS32           0x00300000      0x00300000
  60         Nios II         0x02000000      0x02000000
  61
  62    For example, the "hello world" application may be loaded and
  63    executed on a PowerPC board with the following commands:
  64
  65    => tftp 0x40000 hello_world.bin
  66    => go 0x40004
  67
  68 5. To export some additional function foobar(), the following steps
  69    should be undertaken:
  70
  71    - Append the following line at the end of the include/_exports.h
  72      file:
  73
  74         EXPORT_FUNC(foobar)
  75
  76    - Add the prototype for this function to the include/exports.h
  77      file:
  78
  79         void foobar(void);
  80
  81    - Add the initialization of the jump table slot wherever
  82      appropriate (most likely, to the jumptable_init() function):
  83
  84         gd->jt[XF_foobar] = foobar;
  85
  86    - Increase the XF_VERSION value by one in the include/exports.h
  87      file
  88
  89 6. 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.
  97
  98    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.