I get asked a lot about the process of porting games, so here you go.
When we were at Loki, we worked out of CVS. It was Linux-friendly, obviously, and the price was certainly right. There were other tools out there, like SourceSafe, but they were out of our reach or impractical. So CVS it was.
If you started programming after the beginning of the Subversion epoch, you might not be aware of an interesting quirk of CVS: it doesn't store atomic changesets. That is: when you commit to Subversion, Perforce, git, Mercurial, or any other number of modern tools, you're not committing a bunch of changes to files so much as you are committing a single unit of work, like "fixed this bug," or "added this feature," or "changed all the tabs to spaces."
That wasn't how CVS worked. It kept revisions per-file, and didn't keep much concept of its relation to other files. As such, some things we take for granted, like "show me everything we did this week" wasn't as easy as looking at the shortlog, and things like bisecting were completely out of the question.
For porting, your single unit of work, at commit time--especially at the start of the project--is what I refer to as the "BMW." Lots of commits that touch lots of possibly-unrelated files, all with a commit message that reads simply "Bunch More Work." I made it through three more files and it took an hour? Bunch More Work. Reworked all the templates and removed that nasty macro all over the codebase? Bunch More Work. Ready to go to sleep for the night and everything is different than it was this morning and you don't even remember what? That's a classic Bunch More Work.
This isn't normally acceptable practice, but for porting, there isn't usually a direct set of changes you made precisely--there will be later, when the thing is mostly working and you're applying spackle to all the cracks--and you aren't usually working with anyone else anyhow and if you had to look 100 revisions in either direction it's just an unbuildable wasteland anyhow.
So, between CVS's inability to document complicated changes and our inability to articulate them, the people I worked most with at Loki had a protocol we followed: we wrote changelogs by hand, like diary entries.
This wasn't a new idea at Loki; the GNU project demanded this as protocol for any contributions they accepted, probably because of CVS's predecessor RCS, or maybe because of the nothing that predated that. Surely there was a time when revision control was just Richard Stallman uploading whatever was in his home directory that day to some FTP server, right?
GNU changelogs even have a formal protocol, well documented. In practice, they look like this.
The GNU form was very succinct: quickly jot down what file you changed and why, and since you probably did related work on the same day, you get a reasonable grouping that simulated the Single Unit of Work. But the GNU project was generally setting out to do a Single Unit of Work on any given day, so this made more sense for them. At Loki, we were always at the start of another porting project, and almost everything we did was a big messy pile of Bunch More Work.
Bernd Kreimeier was the master of the Loki changelog, and he insisted every day that I write my own, too, because you can't write it later. That information evaporates into the air. So I didn't just write what I changed in the source code, I wrote what I was planning, what my upcoming concerns were, what I tried that didn't pan out, other things you had to do on the project outside of revision control, like the time you spent rebuilding a Win95 box from scratch to see what a game was supposed to be doing. It would have been a good example of what you'd say at an Agile Scrum Standup thing, if that was a thing we did.
If Richard Stallman was a changelog Hemingway, writing terse, direct prose, I was a changelog Kerouac, writing out stream-of-consciousness on an endless scroll of paper.
When I left Loki and eventually started working on Serious Sam, I kept this habit, even though the CVS repository was just a directory in my home dir.
Eventually CVS moved off my box to a server on the LAN, then became Subversion, and then became Mercurial, and then there was no immediate need to write those massive diary-like changelogs. It's certainly more convenient now. But blowing the dust off of Serious Sam 15 years later and slamming two codeforks together, I was grateful to have this document to see what I did and, crucially, see why I did it.
I'm posting that changelog now. I consider it to be a nice piece of Linux gaming history, so I hope you enjoy it, even as dense as it is to read. But it's also almost 10,000 words about how to port games in the same way that Zen and the Art of Motorcycle Maintainance is accidentally a manual on how to debug software.
Please note that I was 24 when I wrote this, and even though I should have been a grown-ass man, 24 year olds say a lot of cringey things that we think we stopped saying at 14. There's definitely moments in here where I'm certainly scolding Croteam for various design decisions, shaking my fists at the clouds, while those clouds were just trying to ship a game the best they could and feed their families. Fundamentalism and righteous frustration are tones we don't drop until it's way too late. This just happened to be the hill I died on.
Also, as a form of autobiography, you can see when I started to sputter out. I had fixed the big problems, like it not running at all, and was at the point where every problem would take longer and longer to attack for less and less return on that investment. And then, I stopped taking notes. You can see it end after four months of silence, not with a bang but a whimper, on April 24th, 2002, when I wrote the date, no notes, and left a few sad wishlist bugs at the bottom, probably never revisted. By this point, all the notes would have been me banging my head against the wall, trying a million unsuccessful things, cursing my limited tools and time and skills and maybe my own mortality, and just eventually saying fuck it.
So the final unwritten advice at the end of that guide might be: don't give up, always finish, ask for help, and don't let the work consume you. There can always be patches later. Learn what you can, check your assumptions, do better next time and don't beat yourself up too much. Sleep a little sometimes.
The Serious Sam changelog is over here. I hope it helps.