[U-Boot] Weak symbols: request for comments

[Sebastien Carlier <sebastien.carlier@gmail.com>]

Hello all,

I am looking for comments on the use of weak symbols in u-boot.

Some context: u-boot uses weak symbols in several places to provide
default definitions intended to be overriden in individual boards;
this feature is broken with recent toolchains (at least gcc 4.4.4,
binutils 2.20.1), and as a result only the default definitions are
used, while board-specific definitions are silently discarded.

The problem seems to arise because the weak definitions are seen by
the linker before the board-specific ones.  The linker will not look
in the board-specific library archive for strong symbols that would
override already defined weak symbols (this behavior is the one
specified by the System V gABI, so it is correct).

So, U-boot needs to be fixed.  I can see the following ways forward:

1.1) Stop using weak symbols; use pre-initialized function pointers
      instead (possibly grouped in a struct, for cleanliness).
      This has the benefit of offering a clear interface and being
      independent of toolchain details.

1.2) Use regular (non-weak) extern declarations for overridable stuff;
      collect all default weak symbols into a separate library archive,
      to be supplied last to the linker.

1.3) Stop using a library archive for the board specific stuff.
      Instead, collect and link all the object files to produce the
      output binary.  Only Makefile changes are involved, but correct
      behavior depends on all boards doing the right thing.

1.4) Link u-boot into a board-agnostic dynamic library, link the
      board-specific stuff into an executable embedding a dynamic
      linker, and package all this stuff somehow.

Are there better options?  Which one would you prefer to see
implemented?

For reference, here is the list of the definitions currently marked
weak in the u-boot code:

         _machine_restart
         arch_memory_failure_handle
         arch_memory_test_advance
         arch_memory_test_cleanup
         arch_memory_test_prepare
         board_hwconfig
         board_nand_init
         board_reset
         cpu_hwconfig
         do_bootelf_exec
         do_go_exec
         getDebugChar
         kgdb_flush_cache_all
         kgdb_flush_cache_range
         kgdb_interruptible
         kgdb_serial_init
         mg_get_drv_data
         putDebugChar
         putDebugStr
         read_fifo
         spi_cs_activate
         spi_cs_deactivate
         spi_cs_is_valid
         system_map

-- 
Sebastien Carlier