The goal is to get a full Linux environment running on minimal power in something like a mobile phone package.
Q: What's a full Linux environment? A: For the purposes of this discussion, we'll say it's a well supported distribution running with a conventional window manager, good enough CPU, RAM, disk, screen, keyboard and mouse to do normal day-to-day task including development.
Q: What is minimal power? A: For the purposes of this discussion it is 5W, that is 1A at 5V. Why? Because that's the sort of power you can get from conventional Power Packs, so enabling long off-grid usage.
Q: What is good enough CPU? A: From playing with the full range of Raspberry Pis, anything fairly recent seems good enough. Specifically, a single core running at 1GHz is good enough.
Q: What is good enough RAM? A: At 512MB things can swap, 1MB or more is good.
Q: What is good enough disk? A: Enough to install a fully featured environment and have space to play. Let's say that is at least 32GB, preferably 128GB.
Q: What is good enough screen? A: I have 3.5“ going, it's too small! 5” is probably minimally acceptable, anything over 7“ is probably going to take too much power
Q: What is good enough keyboard/mouse? A: Any conventional bluetooth keyboard is probably good enough.
It is commonly accepted that:
They are fairly easy to set up, and they sit nicely over a model B. Rasbian has defaults for small screens that are a good starting point. However, the screen size is really small, it's a real strain to read and too small to do any work. The screen update can be slow, perhaps because it's via the GPIO.
The standard 5” is HDMA, such as this for £23.78. The resolution is 800×480 which is just about acceptable, given all the constraints. I found the power consumption quite high, but I don't have hard numbers yet.
A basic kindle has a 6“ eInk screen, potentially big enough. The refresh rate on eInk is very poor (as it the colour) but it does have lower power consumption. However, the oft quoted “30 days” of battery life for a Kindle is a misconception, it's 30 * 30 mins, or 15 hours. The Kindle itself isn't powerful enough (not enough RAM or disk) and it's really not clear that the extra life of eInk is worth the extra processor and comms and display slowness. Here is what it looks like when working http://netbuffalo.doorblog.jp/archives/4609419.html
“apt install xrdp” on the Raspberry Pi and “RD Client” on Android gives a full independent desktop on Android. However, it transmits bit images which are slow over WiFi.
I picked up a paperwhite 3 (7th generation) on eBay for £30 so this had to be worth a go. The main instructions are here. They are pretty confusing as they are old and outdated, what's more the old system images have been removed. However, for the paperwhite 3 there are other instructions here which are summarised here. This gets you to a rooted paperwhite running the latest software (currently 22.214.171.124).
I haven't yet had time for KUAL, terminal, USB IP hack and then the Debian image…
The best app depends on what version of Android is running. For modern Android (currently 9) https://userland.tech/ seems to be best. As stands, it's not a full distribution, just a chroot environment for the standard Android kernel to make it look like a full Linux. Also there seems to be the need to use a remote desktop app (such as VNC) to see a full desktop. I haven't ruled this out yet, but it would seem to need a lot more work.
Whilst Android is based on the Linux kernel, from what I've found it's near impossible to get a custom kernel to boot, and if it does it is very unlikely to support all the hardware as is needed (e.g. USB, WiFi, touchscreen). I've given up on this unless I stumble across a completely working system.
Some tablets run a x86 processor, commonly the baytrail chipset. Linx and Acer seem to be the affordable choices:
a 8” screen is 30% bigger in area than an 7“ and will presumably use 30% more power. The 8” Linx is more than proportionally heavier, probably due to battery.
I mocked up the 7“ Linx using xrdp and a Raspberry Pi. Screen size was acceptable, refresh was terrible but that was xrdp over WiFi.
7” is big for “mobile phone” size, but having everything in one light, compact and robust container is an improvement over the Raspberry Pi solutions I started with. At £38 including delivery I can't go too far wrong.
./isorespin.sh -i xubuntu-18.04.2-desktop-amd64.iso –atom
mount 126.96.36.199:/home/tonyr /mnt
dd if=/dev/mmcblk1 | gzip > /mnt/mmcblk1.img.gz
That should get you to running system. If it doesn't other useful resources are http://www.linxtablet.co.uk/viewtopic.php?t=2784 cd-rw.org and techradar. Maybe of interest are old notes from Feb 2015.
Suspend to swap would be cool.