- For gcc options, inspect `tmk_core/tool/chibios/chibios.mk`. For instance, I enabled `-Wno-missing-field-initializers`, because TMK common bits generated a lot of warnings on that.
- USB string descriptors are messy. I did not find a way to cleanly generate the right structures from actual strings, so the definitions in individual keyboards' `config.h` are ugly as heck.
- There are some random constants left so far, e.g. 5ms sleep between calling `keyboard_task`, or 1.5sec wait for USB init, in `main.c`. There should be no such in `usb_main.c` (the main USB stack). Everything is based on timers/interrupts/kernel scheduling (well except `keyboard_task`), so no periodically called things (again, except `keyboard_task`, which is just how TMK is designed).
- It is easy to add some code for testing (e.g. blink LED, do stuff on button press, etc...) - just create another thread in `main.c`, it will run independently of the keyboard business.
- Jumping to bootloader works, but it is not entirely pleasant, since it is very much MCU dependent. The code is now geared towards STM32 chips and their built-in bootloaders. So, one needs to dig out the right address to jump to, and either pass it to the compiler in the `Makefile`, or better, define it in `<your_kb>/bootloader_defs.h`. Also, a patch to upstream ChibiOS is needed (supplied), because it `ResetHandler` needs adjusting.
- eeprom / bootmagic (will be chip dependent; eeprom needs to be emulated in flash, which means less writes; wear-levelling?) There is a semi-official ST "driver" for eeprom, with wear-levelling, but I think it consumes a lot of RAM (like 2 pages, i.e. 1kB or so).
- For breathing the caps lock LED during the suspended state ("sleep LED"), it is desirable to have that LED on a hardware PWM pin (there's usually plenty of those, look for TIMERs in the datasheet). However this is not strictly necessary, because instead of direct output of a timer to a pin (better of course), it is easy to define timer callbacks in ChibiOS that turn on/off an arbitrary pin.
- There is also support for AVR8, but the USB stack is not implemented for them yet, and also the kernel itself takes about 1k of RAM. I think people managed to get ChibiOS running on atmega32[8p/u4] though.
- There is some support for K20x and KL2x Freescale chips (i.e. Teensy 3.0, mchck, FRDM-KL25Z, FRDM-K20D50M), but again, no official USB stack yet. This is being worked on, see the `kinetis` branch of [my ChibiOS fork](https://github.com/flabbergast/ChibiOS). It supports also Teensy LC, 3.1 and FRDM-KL26Z.
