Suspend, resume, UEFI, CSM, drm-stable-kmod and drm-next-kmod with Radeon HD 7570M

HP EliteBook 8570p with AMD 'Thames' Radeon HD 7570M.

If neither drm-stable-kmod nor drm-next-kmod is used – commenting out
# kld_list="/boot/modules/radeonkms.ko"
in /etc/rc.conf
and if boot is pure UEFI, without CSM,
then the notebook can reliably resume from suspend. There's a
distinctive single amber pulse of the (normally blue) radio button
before suspend occurs. However:

- without CSM, most of the startup routine is illegible, 'torn'

– for example, I can't see what's typed when I boot to single user mode.

----

If either drm-stable-kmod or drm-next-kmod is used
and if boot is pure UEFI,
then the notebook can not suspend. No amber pulse of the radio button.

With and without drm-next-kmod:
if boot is hybrid UEFI with CSM,
then suspend occurs, but resume fails. No beep, the computer restarts.

debug.acpi.resume_beep=1
in /boot/loader.conf for an audible beep.

----

Please: might graphics/drm-devel-kmod be better for either the tearing (without CSM) or for suspend?

----

$ date ; uname -v
Wed 22 Aug 2018 09:51:39 BST
FreeBSD 12.0-ALPHA2 #2 r337986: Fri Aug 17 22:01:23 BST 2018     root_at_momh167-gjp4-hpelitebook8570p-freebsd:/usr/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG
$ pkg info graphics/drm-stable-kmod
drm-stable-kmod-g20180802
Name           : drm-stable-kmod
Version        : g20180802
Installed on   : Wed Aug 22 06:43:35 2018 BST
Origin         : graphics/drm-stable-kmod
Architecture   : FreeBSD:12:amd64
Prefix         : /usr/local
Categories     : graphics kld
Licenses       : BSD2CLAUSE, MIT, GPLv2
Maintainer     : jmd_at_FreeBSD.org
WWW            : https://github.com/FreeBSDDesktop/kms-drm
Comment        : DRM modules for the linuxkpi-based KMS components
Options        :
        DEBUG          : off
Annotations    :
        FreeBSD_version: 1200078
        repo_type      : binary
        repository     : FreeBSD
Flat size      : 7.51MiB
Description    :
amdgpu, i915, and radeon DRM modules for the linuxkpi-based KMS components.
Currently corresponding to Linux 4.9 DRM. More stable state. amdgpu and
radeonkms are known to fail with EFI boot.

WWW: https://github.com/FreeBSDDesktop/kms-drm
$ pciconf -lv | grep -A 4 vga
vgapci0_at_pci0:1:0:0:     class=0x030000 card=0x17a9103c chip=0x68411002 rev=0x00 hdr=0x00
    vendor     = 'Advanced Micro Devices, Inc. [AMD/ATI]'
    device     = 'Thames [Radeon HD 7550M/7570M/7650M]'
    class      = display
    subclass   = VGA
$