Cyclone5 ethernet through FPGA unstable (Kernel 4.14.130-ltsi)

I have a similar Problem. Custom board with Cyclon5. Quartuns 14.1 used for hardware design. Preloader and u-boot genrated with 14.1 (u-boot 2013.1). Using Yocto Project like described on rocketboards i can generate a rootfs. My Kernel was alswas 3.10-ltsi-rt which wors fine with yocto 2.0 and yocto 2.7 e.g. When i compile a new kernel now (>4.x) and replace my existing kernel i have problem with eth0.

while boot searial terminal shows something like:

stmmaceth ff700000.ethernet eth0: stmmac_open: Cannot attach to PHY (error: -19)

ifconfig just shows loopback interface.

ip link show shows also eth0:

eth0: <BROADCAST,MULTICAST8000> mtu 1500 qdisc noop qlen 1000

link/ether 2e:76:4d:e6:80:14 brd ff:ff:ff:ff:ff:ff

When running ip link set up eth0 i get following error:

stmmaceth ff700000.ethernet eth0: stmmac_open: Cannot attach to PHY (error: -19)

ip: SIOCSIFFLAGS: No such device

Using a newer u-boot than 2013 is also problematic cause then my board does not even boot. I thought that i maybe need to regenerate the preloader and dtb file and so on. But i got some issues there as well therefore i try to run with u-boot 2013 further.

So if you have a solution for your problem maybe it could also resolve mine. I have tried 5-10 different kernels >4.x. Kernel up to <4.0 work fine.

Thanks in advance.

Best regards