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Comment on the makerom linker layout in mkldscript

This commit is contained in:
Tharo 2025-09-09 00:04:50 +01:00
parent 92db06611c
commit c4b38b98bc
1 changed files with 30 additions and 0 deletions
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30
tools/mkldscript.c
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@ -29,6 +29,36 @@ static void write_ld_script(FILE *fout, uint32_t entrypoint_addr, const char *ma
"{\n",
fout);
// Here we write the makerom segment in multiple parts, excluding it from the spec. Before, we would set a fake
// base address of (0x80000400 - 0x1000) so that entry.o, the only case that matters, would end up at the correct
// address. However, this is incompatible with converting the .elf to the ROM image via objcopy due to a bug in how
// binutils computes LMAs from elf sections. Using 0x7FFFF400, the LMA is somehow computed as equal to the VMA:
// Sections:
// Idx Name Size VMA LMA File off Algn
// 0 ..makerom 00001060 7ffff400 00000000 00009400 2**4 CONTENTS, ALLOC, LOAD, RELOC, CODE
// 1 ..makerom.bss 00000000 80000460 80000460 0000a460 2**4 ALLOC
// 2 ..boot 00011f10 80000460 80000460 0000a460 2**5 CONTENTS, ALLOC, LOAD, RELOC, CODE
// 3 ..boot.bss 00004a30 80012370 80012370 0001c370 2**4 ALLOC
// 4 ..dmadata 000060c0 80016da0 00012f70 0001cda0 2**4 CONTENTS, ALLOC, LOAD, RELOC, DATA
// 5 ..dmadata.bss 00000000 8001ce60 8001ce60 0359edf0 2**0 CONTENTS
// 6 ..Audiobank 0002bdc0 00000000 00019030 00023000 2**4 CONTENTS, ALLOC, LOAD, RELOC, CODE
//
// 0x7FFFF400 trips this bug likely due having a different 32-bit sign extension than the other addresses involved.
// Notably, llvm-objdump does not have the same problem with a base address of 0x7FFFF400:
// Sections:
// Idx Name Size VMA LMA Type
// 0 00000000 00000000 00000000
// 1 ..makerom 00001060 7ffff400 00000000 TEXT
// 2 .rel..makerom 00000040 00000000 00000000
// 3 ..makerom.bss 00000000 80000460 00001060 BSS
// 4 ..boot 00011f10 80000460 00001060 TEXT
// 5 .rel..boot 00005f60 00000000 00000000
// 6 ..boot.bss 00004a30 80012370 00012f70 BSS
// 7 ..dmadata 000060c0 80016da0 00012f70 DATA
//
// To simplify things, we simply fix the contents of makerom as they should not change anyway, and assign the
// address only when it matters. The ROM symbols for makerom must encompass these sections only as dmadata would
// not match otherwise.
fprintf(fout,
" /* makerom */" "\n"
"" "\n"