special hardware 4 serials : speed wrong

We have an Odroid C2 with a rather special configuration here.
We need more serial lines so in order to get them, we edited the /boot/meson64…dtb → dts , edited some sections, converted it back to dtb and DietPI’s kernel said “everything okay”.
Our special hardware requirements are as follows :
i2c-b is used for the hardware clock.
4 serial lines are ttyS0,1,2,3 ; two of them are normal serial lines and additionally, b_uart and c_uart is being used to gain two more serial lines.
PREVIOUSLY we have used Ubuntu 18.04.3 LTS with kernel 3.16.72-46 , and aarch64, our own hardware config works there, but other things did not go as smooth as expected.
In DietPI, we have kernel 3.16.57+

Our problem is as follows : On the DietPI our own software tries to communicate with some serial devices with a speed of 500 kilobit/sec, but it fails to do so. The kernel automatically switches to 40 kilobit/sec for some unknown reason. We measured this with the oscilloscope.
The kernel does not tell us why it slows the serial interface down, there is no bug in the log file like the “dmesg” kernel log etc.
But, as I said, we can see it on the oscilloscope that the serial interface is slowed down to 40 kbit/sec.

We suspect that we need a higher kernel version in order to make the serial interface work as fast as required.
Is our assumption correct? If yes, how can we get this to work inside DietPI ?

I am very sorry, please ignore or delete this thread,

We re-installed the “DietPI”-Distro on our Odroid C2 today, and suddenly the 4 serial lines were working with any speed nearly out-of-the-box! All we did was to convert/edit the meson64…dtb file and convert it back and copy it to /boot directory.