99 lines
3.7 KiB
Text
99 lines
3.7 KiB
Text
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, MIPS, Blackfin and Nios II
|
|
architectures use a dedicated register to hold the pointer to the
|
|
'global_data' structure: r2 on PowerPC, r8 on ARM, k0 on MIPS,
|
|
P3 on Blackfin and gp on Nios II. 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
|
|
Blackfin 0x00001000 0x00001000
|
|
Nios II 0x02000000 0x02000000
|
|
|
|
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.
|