Hi... documentation I could find about the signature finder in the tools directory was sort of scarce, so I wrote this while fiddling with it a bit. I had my mind set on adding new features, not porting new cameras:
Signature finder (finsig/gensig) usage:
1) Put all firmware dumps of every camera into the CHDK source tree. Each firmware version needs to be called PRIMARY.BIN and be located in it's own subdirectory, such as platform/a570/sub/100e/PRIMARY.BIN for the a570is 1.00e dump.
Once you have done that, signatures will be automatically searched for during the build process.
You should try batch-build at this point, because it's likely that some platforms will not build due to missing stubs. This is because your dumps may not be perfect (incomplete or not starting from the correct address) or because someone changed something in CHDK source tree that breaks building with firmware dumps in place. If you have a bad firmware dump, fix it (removing garbage from the start is often enough; compare to dumps of other similar cameras and you'll see how it should start) or remove it from the source tree (to skip signature search for this platform), or just don't build for that platform (commenting it out in the Makefile works).
Tweak until the build process goes through cleanly.
2) Go CHDK source tree tools/ directory. See readme.txt. At the time of writing this, it says:
sig_ref_vxworks_1.bin : should contain a610-100e primary
sig_ref_vxworks_2.bin : should contain a710-100a primary
sig_ref_vxworks_3.bin : should contain a570-100e primary
sig_ref_dryos_1.bin : should contain a720-100c primary
Copy those firmware dumps to the tools/ directory with those names.
3) See sig_ref_*.txt. These are simple lists with function names and their entry points for each of these four reference firmware dumps.
Using a disassembler, find entry points to your new functions for each of these four reference firmwares and add them to the end of those files.
4) Build.
If all goes perfectly, your new stubs are now found as NSTUBs in plaform/*/sub/*/stubs_entry.S files among all other the automatically generated stubs.
It's likely that some platforms will not build. The signatures may not be good enough to find your function from all cameras. Your function may not even exist on all cameras, or versions of it may be too different to match the signatures.
To get past an error, you can of course delete the unlucky PRIMARY.BIN or skip building for this particular firmware, but that won't fix the fact that your new signature breaks building for this platform if the firmware dump is in it's place.
What you should do is manually find the entry point for this platform and add it as an NHSTUB to stubs_entry_2.S. If it is not found, you need to add it as a null stub (a stub that has a correct name, but which points to a part of code which will just return without doing anything) to stubs_entry_2.S to keep 'make' happy.
The problem with this approach is that sometimes you may not want to bind this function to a nullstub. Instead, you may want to implement a dummy function in lib.c (only for platforms that don't have the real firmware function) that returns something more sane than a null function. But you can't add that dummy function if the name is taken by the stub!
The alternative is not to add your new function signatures to CHDK svn, but instead provide NHSTUBs in stubs_entry_2.S for all platforms, with dummy stubs or lib.c dummy functions for all unsupported platforms. This doesn't help people porting for a new camera, though (quite the contrary since there's more work to be done in porting).
If the function exists but isn't found, you could find it manually and add this firmware as another new reference firmware, so that this signature would be created and matched. I have never done this, but if I'm not mistaken this is how it's done:
1) Create a new signature input file (for example tools/sig_ref_vxworks_4.txt if that's not taken and if this camera was a vxworks camera) containing your function name and entry point, just like in the other txt files. There is no need to add any other entry points in this file, but for the sake of porting new cameras, it would probably be a good idea to add functions that are listed in stubs_entry_2.S, because those are the ones that weren't found automatically using the old signatures.
2) Copy the firmware dump where gensig can find it (in our example, tools/sig_ref_vxworks_4.bin).
3) Modify tools/Makefile in the numerous occasions where additions are required ('clean:' and 'signatures_vxworks.h:' need to be modified in our example, and 'sig_ref_vxworks_4.bin:' must be added).
4) Update tools/readme.txt.
