Backlight Issues with AMD Radeon

[nicoleprescottaz]

Hello. This is my first time posting, I hope I've chosen the right section.

I have an HP Pavilion DV6-6c35dx running Linux Mint Mate 18.3 Sylvia. inxi -Fzx output:

System: Host: DarkRosePhnx Kernel: 4.13.0-38-generic x86_64 (64 bit gcc: 5.4.0)
Desktop: MATE 1.18.0 (Gtk 3.18.9-1ubuntu3.3) Distro: Linux Mint 18.3 Sylvia
Machine: System: Hewlett-Packard (portable) product: HP Pavilion dv6 Notebook PC v: 0690210000204710000620100
Mobo: Hewlett-Packard model: 1805 v: 33.56 Bios: Hewlett-Packard v: F.17 date: 03/13/2012
CPU: Quad core AMD A8-3520M APU with Radeon HD Graphics (-MCP-) cache: 4096 KB
flags: (lm nx sse sse2 sse3 sse4a svm) bmips: 12776
clock speeds: max: 1600 MHz 1: 1073 MHz 2: 1155 MHz 3: 1053 MHz 4: 1266 MHz
Graphics: Card: Advanced Micro Devices [AMD/ATI] BeaverCreek [Radeon HD 6620G] bus-ID: 00:01.0
Display Server: X.Org 1.18.4 drivers: ati,radeon (unloaded: fbdev,vesa) Resolution: 1366x768@60.04hz
GLX Renderer: AMD SUMO (DRM 2.50.0 / 4.13.0-38-generic, LLVM 5.0.0)
GLX Version: 3.0 Mesa 17.2.8 Direct Rendering: Yes
Audio: Card-1 Advanced Micro Devices [AMD] FCH Azalia Controller driver: snd_hda_intel bus-ID: 00:14.2
Card-2 Advanced Micro Devices [AMD/ATI] BeaverCreek HDMI Audio [Radeon HD 6500D and 6400G-6600G series]
driver: snd_hda_intel bus-ID: 00:01.1
Sound: Advanced Linux Sound Architecture v: k4.13.0-38-generic
Network: Card-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller
driver: r8169 v: 2.3LK-NAPI port: 3000 bus-ID: 01:00.0
IF: eno1 state: down mac: <filter>
Card-2: Realtek RTL8188CE 802.11b/g/n WiFi Adapter driver: rtl8192ce port: 2000 bus-ID: 02:00.0
IF: wlo1 state: up mac: <filter>
Drives: HDD Total Size: 120.0GB (46.2% used) ID-1: /dev/sda model: CT120BX300SSD1 size: 120.0GB temp: 41C
Partition: ID-1: / size: 107G used: 49G (48%) fs: ext4 dev: /dev/sda1
ID-2: swap-1 size: 3.73GB used: 0.00GB (0%) fs: swap dev: /dev/sda5
RAID: No RAID devices: /proc/mdstat, md_mod kernel module present
Sensors: System Temperatures: cpu: 44.0C mobo: N/A gpu: 44.0
Fan Speeds (in rpm): cpu: N/A
Info: Processes: 165 Uptime: 58 min Memory: 941.5/3419.6MB Init: systemd runlevel: 5 Gcc sys: 5.4.0
Client: Shell (bash 4.3.481) inxi: 2.2.35

I am aware that AMD is not offering the proprietary drivers for this version of Linux, although it did not stop me trying to install Catalyst 15.9 (which resulted in a fresh install of LM after getting repeated black screen+blinking cursor and "no OS found" messages on boot - oops). I have been trying to find a work-around for it on the backlight issues. Here's what's happening:

My brightness keys F2 + F3 (and the brightness slider on the applet panel) are working, as in the slider moves (but the backlight does not adjust). As soon as I decrease brightness, the backlight shuts off entirely. I can barely make out the edges of the bottom panel, so I know the screen is still on, but completely unusable. When I turn the brightness back up to max, the screen comes back on normally.

I have tried xbacklight - "no output property"

I have searched for acpi_video and sys/class/backlight - neither seem to even exist.

I have tried sudo find /sys/ -type f -iname '*brightness*'
with the output of:
/sys/devices/pci0000:00/0000:00:06.0/0000:03:00.1/leds/mmc0::/max_brightness
/sys/devices/pci0000:00/0000:00:06.0/0000:03:00.1/leds/mmc0::/brightness
/sys/devices/pci0000:00/0000:00:01.0/backlight/acpi_video0/max_brightness
/sys/devices/pci0000:00/0000:00:01.0/backlight/acpi_video0/brightness
/sys/devices/pci0000:00/0000:00:01.0/backlight/acpi_video0/actual_brightness
/sys/devices/pci0000:00/0000:00:01.0/drm/card0/card0-LVDS-1/radeon_bl0/max_brightness
/sys/devices/pci0000:00/0000:00:01.0/drm/card0/card0-LVDS-1/radeon_bl0/brightness
/sys/devices/pci0000:00/0000:00:01.0/drm/card0/card0-LVDS-1/radeon_bl0/actual_brightness
/sys/devices/platform/i8042/serio0/input/input3/input3::scrolllock/max_brightness
/sys/devices/platform/i8042/serio0/input/input3/input3::scrolllock/brightness
/sys/devices/platform/i8042/serio0/input/input3/input3::capslock/max_brightness
/sys/devices/platform/i8042/serio0/input/input3/input3::capslock/brightness
/sys/devices/platform/i8042/serio0/input/input3/input3::numlock/max_brightness
/sys/devices/platform/i8042/serio0/input/input3/input3::numlock/brightness
/sys/devices/virtual/leds/hp::hddprotect/max_brightness
/sys/devices/virtual/leds/hp::hddprotect/brightness
/sys/module/video/parameters/brightness_switch_enabled

There is no backlight listed so I cannot modify or move or link or anything.

I have tried booting from the USB. No change.

I am curious, as this seems to be generally an Nvidia problem, and I have no Nvidia hardware. I have the AMD Radeon 6620G. So why is everything I find on this problem about Nvidia (and to a much, much smaller degree, Intel)?

I have done a fresh install. I have googled this for hours. I see the terminal in my sleep now. I'm open to suggestions from anyone. I am on the verge of reinstalling Windows, but I really really like LM now that I've gotten to know it, and I have a deep affinity for all things open source.

Help?