xref: /openbmc/u-boot/doc/README.standalone (revision 27b207fd0a0941b03f27e2a82c0468b1a090c745)

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