I have been looking into this myself, with no conclusive information on how to reliably accomplish this. It feels like Qt 5.6 ought to automagically do the scaling for you; I intend to post a new question in this forum asking about this. In the meanwhile, the first part of this video will be of interest to you:

https://youtu.be/nNyhsdX6BsI?t=6m24s

There is code at 06:24 that extracts the dpi value from the Android framework. Maybe that will work better, I have not tried it myself.

@kshegunov I know I know ^^and normally I don't pay much attention to this but.. this time wanted to point it out... dunno why hahahaso as I said np and code happily!! :D

@SGaist
I'm not sure, but I believe solid needs udev. I'm using busybox with mdev.
However, I found a solution with find and grep.
If I know I have to find where, for instance, sda is connected on my 4 ports:

find /sys/bus/usb/devices/usb*/ -name dev -path "*usb1*sd*" | grep -E sda
find /sys/bus/usb/devices/usb*/ -name dev -path "*usb2-1.1*sd*" | grep -E sda
find /sys/bus/usb/devices/usb*/ -name dev -path "*usb2-1.2*sd*" | grep -E sda
find /sys/bus/usb/devices/usb*/ -name dev -path "*usb2-1.4*sd*" | grep -E sda

The return value is 1 if there is no entry matching and 0 if there is.
I wrapped this in a function with sd_ as variable and it works as supposed.

Cheers,
McL

@sierdzio Ok. Thank you a lot.