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https://github.com/AsahiLinux/u-boot
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98 lines
3.6 KiB
Text
98 lines
3.6 KiB
Text
Design Notes on Exporting U-Boot Functions to Standalone Applications:
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======================================================================
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1. The functions are exported by U-Boot via a jump table. The jump
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table is allocated and initialized in the jumptable_init() routine
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(common/exports.c). Other routines may also modify the jump table,
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however. The jump table can be accessed as the 'jt' field of the
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'global_data' structure. The slot numbers for the jump table are
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defined in the <include/exports.h> header. E.g., to substitute the
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malloc() and free() functions that will be available to standalone
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applications, one should do the following:
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DECLARE_GLOBAL_DATA_PTR;
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gd->jt[XF_malloc] = my_malloc;
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gd->jt[XF_free] = my_free;
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Note that the pointers to the functions all have 'void *' type and
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thus the compiler cannot perform type checks on these assignments.
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2. The pointer to the jump table is passed to the application in a
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machine-dependent way. PowerPC, ARM, MIPS and Blackfin architectures
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use a dedicated register to hold the pointer to the 'global_data'
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structure: r2 on PowerPC, r8 on ARM, k0 on MIPS, and P5 on Blackfin.
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The x86 architecture does not use such a register; instead, the
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pointer to the 'global_data' structure is passed as 'argv[-1]'
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pointer.
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The application can access the 'global_data' structure in the same
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way as U-Boot does:
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DECLARE_GLOBAL_DATA_PTR;
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printf("U-Boot relocation offset: %x\n", gd->reloc_off);
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3. The application should call the app_startup() function before any
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call to the exported functions. Also, implementor of the
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application may want to check the version of the ABI provided by
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U-Boot. To facilitate this, a get_version() function is exported
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that returns the ABI version of the running U-Boot. I.e., a
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typical application startup may look like this:
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int my_app (int argc, char *argv[])
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{
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app_startup (argv);
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if (get_version () != XF_VERSION)
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return 1;
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}
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4. The default load and start addresses of the applications are as
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follows:
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Load address Start address
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x86 0x00040000 0x00040000
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PowerPC 0x00040000 0x00040004
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ARM 0x0c100000 0x0c100000
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MIPS 0x80200000 0x80200000
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Blackfin 0x00001000 0x00001000
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For example, the "hello world" application may be loaded and
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executed on a PowerPC board with the following commands:
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=> tftp 0x40000 hello_world.bin
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=> go 0x40004
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5. To export some additional function foobar(), the following steps
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should be undertaken:
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- Append the following line at the end of the include/_exports.h
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file:
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EXPORT_FUNC(foobar)
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- Add the prototype for this function to the include/exports.h
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file:
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void foobar(void);
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- Add the initialization of the jump table slot wherever
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appropriate (most likely, to the jumptable_init() function):
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gd->jt[XF_foobar] = foobar;
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- Increase the XF_VERSION value by one in the include/exports.h
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file
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6. The code for exporting the U-Boot functions to applications is
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mostly machine-independent. The only places written in assembly
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language are stub functions that perform the jump through the jump
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table. That said, to port this code to a new architecture, the
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only thing to be provided is the code in the examples/stubs.c
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file. If this architecture, however, uses some uncommon method of
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passing the 'global_data' pointer (like x86 does), one should add
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the respective code to the app_startup() function in that file.
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Note that these functions may only use call-clobbered registers;
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those registers that are used to pass the function's arguments,
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the stack contents and the return address should be left intact.
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