objdump -S for a different dir.

hi folks, out of habit, I build my kernels to a build tree ie use make O=../build-foo just once, then cd ../build-foo, and thereafter run make there this doesnt play nice with objdump -S --source Display source code intermixed with disassembly, if possible. Implies -d. it seems its not possible unless the source and obj are in the same file. GDB manages to find the source, whats missing in objdump ? FWIW, Id love a micro-tutorial on this, anyone know how to display, dissect the obj file, debuginfo etc ? or can recommend some on-line tome that gets into it at some breadth and depth ? many many search hits are for quick Q&A that doesnt get deeper than a puddle. thanks