radeonkms: kernel crash preceded by RADEON_FLIP_SUBMITTED message

[Graham]

My system is an AMD A10-5700 with 16GB ram and 240GB SSD.  'uname -a' 
shows....
FreeBSD tigger 12.2-RELEASE-p1 FreeBSD 12.2-RELEASE-p1 GENERIC  amd64

I installed drm-fbsd12.0-kmod and drm-kmod from ports (as advised) after 
upgrading to 12.2-RELEASE

I have found I can consistently trigger a crash-and-reboot by running 
glxgears, expanding its window size, then contracting it.

If I add these to /boot/loader.conf

dev.drm.skip_ddb=1
dev.drm.drm_debug=-1
dev.drm.drm_debug_persist=1

there results a massive (~100MB) /var/log/messages file including all 
the radeon diagnostics.

The reboot is always preceded by this message, just once for each crash:

kernel: [drm:radeon_crtc_handle_flip] radeon_crtc->flip_status = 0 != 
RADEON_FLIP_SUBMITTED(2)

This is not the last radeon message, but less than a second after 
contracting the window, the kernel usually crashes.

The stack trace shows (amongst other data which I cannot interpret):

Fatal trap 12: page fault while in kernel mode
current process Xorg

kdb_backtrace+0x65
vpanic+0x17b
panic+0x43
trap_fatal+0x391
trap_pfault+0x4f
trap+0x286
calltrap+0x0
ttm_bo_man_put_node+0x3c
ttm_bo_cleanup_refs_or_queue+0x1fc
ttm_bo_unref+0x81
radeon_bo_unref+0x22
radeon_gem_object_free+0x1e
drm_gem_object_release_handle+0xcb
drm_gem_handle_delete+0x8e
derm_ioctl_kernel+0xf1
drm_ioctl+0x289
linux_file_ioctl+0x318
kern_ioctl+0x2b7
n.b. if the numbers matter here, remember that '8' and '0' characters 
are difficult to distinguish.

Without loading radeonkms module, graphics (I presume without gpu) does 
not crash.  However cannot play video without dropping frames.
If radeonkms is loaded (by /etc/rc.conf or by hand after boot) gpu gets 
used, but it can be crashed using glxgears.

Any suggestions are welcome for a fix, or upgrades I have missed, or how 
to get more informative diagnostics.
cheers
Graham