Ha, I've soldered to the same pins as the author! I just left the wires dangling out of the case, but pulled them through the aluminum latches (?) in the case for some stabilization.
> Final hurdle: the kindle serial port runs at 1.8v, so I needed a serial port adaptor which supports that
It turns out that most adapters support receiving at 1.8V, you just need a simple voltage divider to keep them from sending more.
Unfortunately I didn't really achieve what I wanted to - I'd like to wake the Kindle using some external device that can poll data at lower power (Thread/BLR/ESP Now) to build an E-Ink display with a long battery life and that can be updated with a relatively low latency. It seems like the chip has a feature for being woken up from deep sleep via UART, but it's disabled by default and needs to be set with a kernel parameter and I haven't managed to set one so far (cmdline set in U-Boot keeps being reset to the original one). Maybe someone has an idea? Does my approach even make sense? Could I get away with a similar efficiency with the Kindle's hardware alone?
... if you want most other things, you don't really even need to open the shell, though. There's a huge amount of software-based jailbreaks over on Mobileread along with cool helpful software tools, tips and cool people.
The Kindle is a whole single board computer with a full Linux system (which is a joy to explore by the way, it's largely made up of shell scripts and js, which is often not only not obfuscated, but commented! big components used include upstart, X11, GTK2 and awesome) and you can install fairly modern software on it - though the kernel (2.6 or so) and architecture (armhf) is starting to limit the older versions a lot. Still, I have Python 3.7 (I think) and Syncthing on my Kindle Touch and it's working quite well.
One use case for actually soldering to the pins is that you can build with it with very little messing around with software. My uncle, a tinkerer without any Linux experienced, is considering just using the Kindle as a dumb display to be drawn to, which this approach would be ideal for. Compute the password, log in, draw using the CLI tools or directly to the framebuffer, all without ever pressing a button on the Kindle.
... these devices are cool. Repurposing old cheap consumer hardware is very cool. Though the Kindle Touch is still very capable for reading (with KOReader, of course)!
"really cheap ones marked “BLOCKED BY AMAZON”; I decided not to go for these since theoretically they might have been stolen."
I'm not sure Amazon even has any way to report a stolen device to them so they can block it. I have a blocked Paperwhite(1st gen), because Amazon sent me a new one as a warranty replacement, but they specifically said that I don't need to return the old one - it's just blocked from accessing their servers. I imagine that's exactly what those cheap kindles on eBay are.
Props for the hacking aspect and reuse of a locked device.
If you just wanted to mess with "an" eink device, a Kobo is much easier since it's already just a Linux device and can easily be accessed without hacks.
On at least some models, you can also swap the SD card and start a whole fresh system, so no matter what you're up to, you can keep your ereader too.
And even easier: koreader ships with SSH, all you have to do is install that for wireless console access without changing the SD card.
You might want to consider Pine64's PineNote, which is being developed as a Linux-ready, hackable device. However, development seems to be in its earliest stages: even booting Linux is a serious challenge, and recovering from softbricks is unusually difficult https://wiki.pine64.org/wiki/PineNote For most people, a Kobo will almost certainly be more straightforward.
Kindles are Linux devices too, or at least they were through the end of 2017 when I left a team that worked on them. It seems likely they still are. You could SSH into them over USB if you had the root password, and even internally we used public hacker tools to get those passwords. I'm not positive this is the same one, but it looks familiar. https://www.sven.de/kindle/
No idea, but I assume you can just get into whatever driver interfaces the userspace Kobo software (IIRC, it's QT-based) is using. I can poke at the command line if you want to get some idea of what's there, let me know.
Certainly Koreader has no issues displaying what it wants to (and it does at least some level of e-ink-friendly regional updates).
The PC emulator is worth a go too if you get into Koreader itself. Very handy for shortening the OODA loop over downloading to a target device. And the IRC channel is also useful.
I rooted two rather old kindles and it worked via USB and without any soldering. So maybe if you want to do this, it’s easier to go for a slightly newer model and do it without opening it up?
That's easier if you're into e-paper technology, but they don't sell them for £7...