--- /dev/null
+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:
+
+ * 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.
+
+ * 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.
+
+The following changes will be made:
+
+- The syscall handling code will be removed.
+
+- The include/_exports.h file will be introduced, containing the list
+ of the exported functions in the following form:
+
+ EXPORT_FUNC(getc)
+ EXPORT_FUNC(tstc)
+ ...
+
+ 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.
+
+- The include/exports.h file will be introduced, containing the
+ prototypes of the exported functions and the assigned slot numbers.
+
+- The examples/stubs.c file will be introduced, containing the code
+ for the jump stubs for each of the exported functions.
+
+Implementation Notes on Exporting U-Boot Functions:
+===================================================
+
+1. The patch was applied against TOT as of 7/24 12:50 MEST; the
+ resulting images were tested on the following boards:
+
+ * lwmon (PowerPC)
+ * trab (ARM)
+ * inca (MIPS)
+
+ The hello_world application was loaded and executed then:
+
+ [lwmon]
+ => tftp 0x40000 /tftpboot/LWMON/hello_world.bin-avn
+ => go 0x40004
+
+ [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
projects / oweals / u-boot.git / blobdiff