Design Notes on Exporting U-Boot Functions to Standalone Applications:
======================================================================
-1. Add a field to the global_data structure, the pointer to a jump
- table.
-
-2. Jump table itself is allocated and filled in the same way as the
- syscall table is (allocated with malloc() after the code has been
- relocated to RAM); a special function, fixed to the table element
- number 0, will be added which returns the ABI version so
- applications can check for compatibility issues.
-
-3. It is application's responsibility to check the ABI version and
- act accordingly.
-
-4. Pointer to the global_data is passed to the application in the
- dedicated register that is used in the U-Boot to hold this
- pointer. This assumes that the application is built with the same
- register- allocation flags as the U-Boot itself. (Actually, this
- is a requirement even now, as the 'go' command does not perform
- any actions to protect this register against being clobbered by
- the application).
-
- This approach won't work on the x86 architecture. See below.
-
-5. Application now calls standard library functions like printf()
- instead of specially prefixed names like mon_printf() as it did
- before. Present implementation of these functions (using the
- system calls mechanism) will be replaced with jump stubs.
-
-6. To export additional functions, the following steps will have to be
- taken:
-
- - Add the xxx() U-Boot function to the EXPORT_FUNC list
- - Add initialization of the appropriate slot in the jump table
-
-7. To port to a new architecture, the appropriate stub code should be
- provided. No other machine-dependent code is used. Once the stub
- template is available, no additional coding is needed when
- exporting new U-Boot functions. A pre-processor macro will be used
- to automatically instantiate the stub definition for each exported
- function.
-
-Note the following:
-
-- This approach uses a jump table with fixed slot allocation. That
- said, to retain the ABI compatibility, no table reordering,
- inserting new functions in the middle of the list or deleting
- functions from the list is allowed. Any such action will break the
- ABI compatibility.
-
-- The x86 architecture does not use a dedicated register to store the
- pointer to the global_data structure. There are the following
- approaches available:
+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 struct members 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->malloc = my_malloc;
+ gd->jt->free = my_free;
+
+ Note that the pointers to the functions are real function pointers
+ so the compiler can 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, r9 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
- * Pass the global_data pointer to the application in a register or
- as an additional argument. This requires special machine-
- dependent startup code to be compiled into the application.
+5. To export some additional function long foobar(int i,char c), the following steps
+ should be undertaken:
- * Make the x86 consistent with the rest of architectures and use a
- dedicated register. This renders one register unusable in the
- rest of the U-Boot code and thus increases the size of the U-Boot
- binary and decreases it performance.
+ - Append the following line at the end of the include/_exports.h
+ file:
-The following changes will be made:
+ EXPORT_FUNC(foobar, long, foobar, int, char)
-- The syscall handling code will be removed.
+ Parameters to EXPORT_FUNC:
+ - the first parameter is the function that is exported (default implementation)
+ - the second parameter is the return value type
+ - the third parameter is the name of the member in struct jt_funcs
+ this is also the name that the standalone application will used.
+ the rest of the parameters are the function arguments
-- The include/_exports.h file will be introduced, containing the list
- of the exported functions in the following form:
+ - Add the prototype for this function to the include/exports.h
+ file:
- EXPORT_FUNC(getc)
- EXPORT_FUNC(tstc)
- ...
+ long foobar(int i, char c);
- This list will be used to assign the slot numbers in the jump
- table, to determine the size of the jump table and to generate the
- code for the stub functions.
+ Initialization with the default implementation is done in jumptable_init()
-- The include/exports.h file will be introduced, containing the
- prototypes of the exported functions and the assigned slot numbers.
+ You can override the default implementation using:
-- The examples/stubs.c file will be introduced, containing the code
- for the jump stubs for each of the exported functions.
+ gd->jt->foobar = another_foobar;
-Implementation Notes on Exporting U-Boot Functions:
-===================================================
+ The signature of another_foobar must then match the declaration of foobar.
-1. The patch was applied against TOT as of 7/24 12:50 MEST; the
- resulting images were tested on the following boards:
+ - Increase the XF_VERSION value by one in the include/exports.h
+ file
- * lwmon (PowerPC)
- * trab (ARM)
- * inca (MIPS)
+ - If you want to export a function which depends on a CONFIG_XXX
+ use 2 lines like this:
+ #ifdef CONFIG_FOOBAR
+ EXPORT_FUNC(foobar, long, foobar, int, char)
+ #else
+ EXPORT_FUNC(dummy, void, foobar, void)
+ #endif
- The hello_world application was loaded and executed then:
- [lwmon]
- => tftp 0x40000 /tftpboot/LWMON/hello_world.bin-avn
- => go 0x40004
+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.
- [trab]
- TRAB # tftp 0xc100000 /tftpboot/TRAB/hello_world.bin-avn
- TRAB # go 0xc100000
-
- [inca]
- INCA-IP # tftp 0x80200000 /tftpboot/INCA/hello_world.bin-avn
- INCA-IP # go 0x80200000
-
-2. As neither of supported x86 boards can be built from the TOT
- sources currently, the patch build was verified by manually
- running the following command in the U-Boot top directory:
-
- > make -C examples TOPDIR=`pwd` ARCH=i386 CROSS_COMPILE=
-
- The rest of the code is mostly machine-independent and was not
- verified.
-
-3. To test the x86 assembly code, a small standalone application was
- written. It was built and run on the RedHat Linux 8.0 (x86). The
- application performs a jump using a pointer to jump table and a
- function's index in it.
-
-4. For the MIPS architecture, the linker script is also provided for
- linking applications. The default linker script places the .text
- and .data sections too far from each other so that the resulting
- .bin files span about 256Mb in size.
-
-5. Several example applications required updating for the new API.
- These applications relied upon the bd_t pointer being passed as
- the 1st argument to the main function; this had changed when the
- system calls were introduced, but apparently, these applications
- weren't fixed at that moment. This is fixed now.
-
-6. GCC issues warnings for the 'sched' application. Since now the
- mon_printf() function is renamed to printf(), GCC applies its
- knowledge of the format specifiers to check the arguments,
- complaining about ints passed as longs and vice versa. This is not
- fixed yet.
-
-7. Only the hello_world example application was modified to make use
- of the newly supplied get_version() function. The application now
- prints two ABI versions, the one that the application was compiled
- for and the other, actual ABI version.
-
-8. The following new files were added:
- common/exports.c
- examples/mips.lds
- examples/stubs.c
- include/_exports.h
- include/exports.h
- doc/README.standalone
-
- The following files are no longer used and will be removed:
- examples/syscall.S
- include/syscall.h
+ 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.
projects / karo-tx-uboot.git / blobdiff