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