Error message
xillybus: Failed to allocate write buffer memory. Aborting.Meaning
This fatal error message indicates that the Linux kernel failed to supply a chunk of memory necessary to contain a RAM buffer. Despite the error message's wording, the lack of memory could be result of allocation of memory for either read or write buffers.
This condition is by far not expected to happen with the evaluation kit, since the RAM buffers necessary for it are rather modest in size. Custom cores designed for high bandwidth performance may require many large continuous segments of memory, which may be beyond the kernel's capabilities to supply.
A possible contributor to memory allocation failures can RAM fragmentation in kernel space. The sooner the module is loaded after boot, the better.
Recommended action
If the kernel module was loaded manually, relying on automatic load at boot may work better. A proper installation of the Xillybus driver causes the module to load at boot.
Tweaking the kernel's configuration and/or compilation flags may also be helpful.
Please report this error, as a core reconfiguration may be the solution as well.