Kevin Hilman [off-list ref] writes:

[...]

It seems to be pretty stable up through v4.4 but
unreliable/unpredictable in v4.5-rc1 and lots of places in between.
It's hard to bisect because it doesn't reliably fail, but when it does
fail to finish booting, it's typically after it reaches a shell, and
trying to run various simple shell commands fails to complete, like
this example from next-20160125[2].

To me this suggests possibly an issue with the idle driver where
characters might be getting lost on the serial console, which causes
the automated boot scripts to fail because the shell is still waiting
for the command to be finished.

FWIW, I think I confirmed my suspicion that it's an idle driver issue
because adding 'nohlt' on the cmdline, which forces cpuidle_poll mode
(essentially disabling cpuidle) makes mainline boot quite reliably on my
mt8173-evb board now.

Kevin