--- /dev/null
+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 * const 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
+ NDS32 0x00300000 0x00300000
+ 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.
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