The kindle serial port is 1.8v. A resistive divider will work for the:
PC -> Kindle
But the other direction is far more complex - 1.8v -> 3.3v won't work with the standard mosfet level shifter.
Best bet is to get an adapter which does 1.8v natively - I made one with a FT232rl.
I can confirm that the kernel with initrd works fine to change the password. If the kindle appears "stuck", just hit the power switch for a short time (1s) and it wakes back up.
Cheers,
Allan.
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