imac12,2 (27” 2011) with Radeon HD 6770M can’t change backlight/brightness

I have a sibling of the iMac12 model described in issue #1838 (closed), which was closed (I think successfully) about a year ago.

The title was "imac12,1 (21.5” 2011) with Radeon HD 6750M can’t change backlight/brightness"

What I have here is the iMac12,2 (27" 2011) model, which seems to have a Radeon HD 6770M. Same problem as in the older issue report, there should be a "radeon_bl0" folder in /sys/class/backlight, but when installing openSUSE Tumbleweed with kernel 6.4.6 there is only "acpi_video0", and manipulating the value in the "brightness" file in that folder has no effect on the backlight at all. Max value for that is 15 in the "max_brightness" file, other values cause an error. But no value from 0 to 15 makes any difference in brightness.

Booting from a Linux Mint 21.2 live ISO which is still using kernel 5.15, I see the "radeon_bl0" folder. Max value is 255, and manipulating the value in the "brightness" file with values between 0 and 255 will change the backlight brightness from completely off to full brightness.

Oddly, the brightness keys while booted into Linux Mint 21.2 show the OSD overlay changing, but apparently the backlight device being manipulated by the brightness keys is the "acpi_video0" device, since there is no change in brightness unless I manually change the value in the "radeon_bl0" folder.

Any chance a similar patch can be made to the one that fixed #1838 (closed)? I barely know what I'm looking at but the patch diff seemed like it was only changing a few lines of something.

Edit: I didn't notice there was a way to link a new issue to an existing issue (from the vertical "..." menu). There doesn't seem to be a way to add this linking after the fact by just editing the issue.