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 struct members 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->malloc  = my_malloc;
  16         gd->jt->free = my_free;
  17
  18    Note that the pointers to the functions are real function pointers
  19    so the compiler can 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         RISC-V          0x00600000      0x00600000
  62
  63    For example, the "hello world" application may be loaded and
  64    executed on a PowerPC board with the following commands:
  65
  66    => tftp 0x40000 hello_world.bin
  67    => go 0x40004
  68
  69 5. To export some additional function long foobar(int i,char c), the following steps
  70    should be undertaken:
  71
  72    - Append the following line at the end of the include/_exports.h
  73      file:
  74
  75         EXPORT_FUNC(foobar, long, foobar, int, char)
  76
  77         Parameters to EXPORT_FUNC:
  78          - the first parameter is the function that is exported (default implementation)
  79          - the second parameter is the return value type
  80          - the third  parameter is the name of the member in struct jt_funcs
  81            this is also the name that the standalone application will used.
  82            the rest of the parameters are the function arguments
  83
  84    - Add the prototype for this function to the include/exports.h
  85      file:
  86
  87         long foobar(int i, char c);
  88
  89         Initialization with the default implementation is done in jumptable_init()
  90
  91         You can override the default implementation using:
  92
  93         gd->jt->foobar = another_foobar;
  94
  95         The signature of another_foobar must then match the declaration of foobar.
  96
  97    - Increase the XF_VERSION value by one in the include/exports.h
  98      file
  99
 100    - If you want to export a function which depends on a CONFIG_XXX
 101         use 2 lines like this:
 102         #ifdef CONFIG_FOOBAR
 103                 EXPORT_FUNC(foobar, long, foobar, int, char)
 104         #else
 105                 EXPORT_FUNC(dummy, void, foobar, void)
 106         #endif
 107
 108
 109 6. The code for exporting the U-Boot functions to applications is
 110    mostly machine-independent. The only places written in assembly
 111    language are stub functions that perform the jump through the jump
 112    table. That said, to port this code to a new architecture, the
 113    only thing to be provided is the code in the examples/stubs.c
 114    file. If this architecture, however, uses some uncommon method of
 115    passing the 'global_data' pointer (like x86 does), one should add
 116    the respective code to the app_startup() function in that file.
 117
 118    Note that these functions may only use call-clobbered registers;
 119    those registers that are used to pass the function's arguments,
 120    the stack contents and the return address should be left intact.