radeon board, "fence_wait returned with error"

[Gary Aitken]

Running 11.2 release with a radeon HD 5550 board,

11.2-RELEASE-p4 FreeBSD 11.2-RELEASE-p4 #0: Thu Sep 27 08:16:24 UTC 2018     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

kldstat shows the following drivers:
     radeonkms.ko
     radeon_REDWOOD_pfp_bin.ko
     radeon_REDWOOD_me_bin.ko
     radeon_REDWOOD_rlc_bin.ko
     radeon_REDWOOD_smc_bin.ko
     radeon_CYPRESS_uvd_bin.ko

Things seem to be running fine but I am getting the following errors
in my security log:

+fence_wait returned with error -512

Can anyone explain to me what these are about?

I found this thread:
   http://freebsd.1045724.x6.nabble.com/drm2-in-base-td6284182.html

but it's not clear to me what the fence is and what fence_wait is all
about.

Thanks,

Gary