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Design Notes on Exporting U-Boot Functions to Standalone Applications:
======================================================================
1. The functions are exported by U-Boot via a jump table. The jump
table is allocated and initialized in the jumptable_init() routine
(common/exports.c). Other routines may also modify the jump table,
however. The jump table can be accessed as the 'jt' field of the
'global_data' structure. The slot numbers for the jump table are
defined in the <include/exports.h> header. E.g., to substitute the
malloc() and free() functions that will be available to standalone
applications, one should do the following:
DECLARE_GLOBAL_DATA_PTR;
gd->jt[XF_malloc] = my_malloc;
gd->jt[XF_free] = my_free;
Note that the pointers to the functions all have 'void *' type and
thus the compiler cannot perform type checks on these assignments.
2. The pointer to the jump table is passed to the application in a
machine-dependent way. PowerPC, ARM and MIPS architectures use a
dedicated register to hold the pointer to the 'global_data'
structure: r29 on PowerPC, r8 on ARM and k0 on MIPS. The x86
architecture does not use such a register; instead, the pointer to
the 'global_data' structure is passed as 'argv[-1]' pointer.
The application can access the 'global_data' structure in the same
way as U-Boot does:
DECLARE_GLOBAL_DATA_PTR;
printf("U-Boot relocation offset: %x\n", gd->reloc_off);
3. The application should call the app_startup() function before any
call to the exported functions. Also, implementor of the
application may want to check the version of the ABI provided by
U-Boot. To facilitate this, a get_version() function is exported
that returns the ABI version of the running U-Boot. I.e., a
typical application startup may look like this:
int my_app (int argc, char *argv[])
{
app_startup (argv);
if (get_version () != XF_VERSION)
return 1;
}
4. The default load and start addresses of the applications are as
follows:
Load address Start address
x86 0x00040000 0x00040000
PowerPC 0x00040000 0x00040004
ARM 0x0c100000 0x0c100000
MIPS 0x80200000 0x80200000
For example, the "hello world" application may be loaded and
executed on a PowerPC board with the following commands:
=> tftp 0x40000 hello_world.bin
=> go 0x40004
5. To export some additional function foobar(), the following steps
should be undertaken:
- Append the following line at the end of the include/_exports.h
file:
EXPORT_FUNC(foobar)
- Add the prototype for this function to the include/exports.h
file:
void foobar(void);
- Add the initialization of the jump table slot wherever
appropriate (most likely, to the jumptable_init() function):
gd->jt[XF_foobar] = foobar;
- Increase the XF_VERSION value by one in the include/exports.h
file
6. The code for exporting the U-Boot functions to applications is
mostly machine-independent. The only places written in assembly
language are stub functions that perform the jump through the jump
table. That said, to port this code to a new architecture, the
only thing to be provided is the code in the examples/stubs.c
file. If this architecture, however, uses some uncommon method of
passing the 'global_data' pointer (like x86 does), one should add
the respective code to the app_startup() function in that file.
Note that these functions may only use call-clobbered registers;
those registers that are used to pass the function's arguments,
the stack contents and the return address should be left intact.
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