On the M4 it says: Name service handshake is done, M4 has setup a rpmsg channel [0 ---> 1024], so far OK.
On the A7 side:
Waiting MU transmit buffer empty timeout!
ugh, imx_mu_rpmsg_send() failed: -5
imx_rpmsg_tty rpmsg0: virtqueue_add_outbuf failed: -5
imx_rpmsg_tty rpmsg0: rpmsg_send failed: -5
I think it means that the M4 does not read the messages.
We got it working, there were two issues:
1. We where wrong footed by some remarks in the device tree and therefore changed our linker-file.Now we are using the original linker-file again.
2. The heap was to small, the was not enough space for the communication queues. When moving to ocram we added extra code which resulted in using extra heap space.