On Fri, 22 Dec 2017 13:44:27 +0100
Johan Hovold [off-list ref] wrote:

[...]

I'd suggest reiterating the problem you're trying to solve and
enumerating the previously discussed potential solutions in order to
find a proper abstraction level for this (before getting lost in
implementation details).

The main point here is in short words: Having a device powered on or off
when the uart it is attached to, is used or not used anymore,
so the already available userspace applications do not need to be changed.

I digged out a bit around:
alternative aproaches were:
adding hooks to the uart/tty layer:
https://marc.info/?l=linux-kernel&m=143333222014616&w=2
https://marc.info/?l=devicetree&m=143130955414580&w=2

I do not find it right now in my archive:
adding a virtual gpio for dtr to the omap_serial driver.
The driver behind the virtual io would then handle pm. One reason it was
rejected was that the devicetree should only contain real hardware and
not virtual stuff.

Regards,
Andreas