There seems to be a bug in FreeRTOS BSP V1.0.1 for the iMX6SX.
When running the rpmsg pingpong demo with V1.0.0 the variable pingpongs from 0 to 199999 (100000 iterations) on the A9.
However, when running the same demo with V1.0.1 the variable goes to 999 and is then reset to 0 and then up to 999 again to be reset for its 100000 iterations.
Using a second application on the A9 the problem seems to be located in the receive where 256 is sent from the A9 but the M4 says its received 0. This second application keeps counting till overflow with V1.0.0.