[yocto] [meta-raspberrypi] Unified serial console device option

[Andrei Gherzan]

Hello all,

I'm trying to have the ability for user to use serial0 as serial port
on the header and serial1 (when available) for Bluetooth. In this way
rpi2 and rpi3 (32b) will have the same configuration. The story goes
identical for rpi0 and rpi0-wifi.

Now, what I was planning to do is:
1. create udev rules to generate the the symlinks accordingly to dt aliases.
2. add the above package (with the rules) as a machine recommends
(MACHINE_ESSENTIAL_*).
3. and then change all the SERIAL_CONSOLE = 'serial0 115200'

This works great with one exception: system parses the console
argument in cmdline and spawns through a generator a new getty
instance on that device. Adding systemd-serialgetty package will make
system have two getty services on the same device (serial0 and for
example AMA0) which is obviously unusable and wrong. Sysvinit works
ok.

Then I thought to make this completely optional:
1. create udev rules to generate the the symlinks accordingly to dt aliases.
2. make SERIAL_CONSOLE configurable (default assignments)
3. a user wanting to take advantage of the udev rules will have to add
the rpi udev rules to the image and modify in his/her local.conf
SERIAL_CONSOLE

Currently we have a cascade setup for our machine configuration: ex
rpi3 includes rpi2 and tweaks some things. We do this to avoid some
duplication (even though the gain is really minimal - 2-3 lines).
Anyway it is something after all.

This again looks fine but because we have machine which include other
machine conf, I endded up having a problem when I want to overwrite
both definitions: see rpi3 which includes rpi2 (defines a
SERIAL_CONSOLE) than overwrites SERIAL_CONSOLE.

What do you guys think as a way forward? One way would be to make the
boards definitions be stand alone and use default assignments which I
think it would fix everything and make the mechanism completely
optional.

BTW: This would unlock the ability to create a raspberrypi-all machine
that would work on any raspberry board (armv6 + wifi firmware + etc).

--
Andrei Gherzan