[Linux] Custom baud rate method deprecated on some UART chipset ?

Hi, I finally managed to test some DMX data transmission using QtSerialPort and threads (thanks to kuzulis)

My simple app works fine on a FTDI DMX-USB adapter and now I am testing the same app on a Raspberry Pi with a UART DMX hat:
http://www.bitwizard.nl/shop/index.php?route=product/product&product_id=153

The DMX protocol requires a custom rate of 250Kbps so I call setBaudRate(250000) before opening a QSerialPort.
On the FTDI adapter (ttyUSB0) this works, while on the UART hat (ttyAMA0) it doesn't output anything and the kernel says:
[ 90.322470] uart-pl011 dev:f1: WriterThread sets custom speed on ttyAMA0. This is deprecated.
Kernel is: Linux raspberry-pi 3.18.11-v7+ #777 SMP PREEMPT Sat Apr 11 17:30:37 BST 2015 armv7l GNU/Linux
Libc version is: 2.13-38+rpi2+deb7u8

Searched for the message with google and I found a couple of interesting discussions:
http://help.lockergnome.com/linux/Serial-custom-speed-deprecated--ftopict368717.html
http://stackoverflow.com/questions/12646324/how-to-set-a-custom-baud-rate-on-linux

Apparently, setting a clock divisor is not always the right way and I can confirm another software using pure C calls works fine with the UART hat that I have. This is how they do it:
https://github.com/OpenLightingProject/ola/blob/master/common/io/ExtendedSerial.cpp#L47

I tried to make a patch with that code into qserialport_unix.cpp but with no luck.

Any clue ?