Re: wsdisplay brightness issue on amd64

On Mon, Aug 10, 2026 at 10:16:59AM +1000, Jonathan Gray wrote:
> On Sun, Aug 09, 2026 at 08:44:45PM +0100, Jason McIntyre wrote:
> > On Sun, Aug 09, 2026 at 11:12:56AM +0100, Jason McIntyre wrote:
> > > hi/
> > > 
> > > when this machine boots, the screen display dims severely right about the
> > > time my wsdisplay attach shows in dmesg. i can fix it manually by
> > > adjusting display.brightness.
> > > 
> > > setting display.brightness to 0 is brighter than the booted default.
> > > 
> > > setting most display.brightness values work as normal, but 97-100 revert
> > > it to this super dim state.
> > > 
> > > i have not updated the laptop in two weeks, so hard to pin down when
> > > this would have happened (it's running current).
> > > 
> > > i confirmed this happened on a boot of stock /bsd.mp.
> > > 
> > > can;t think of anything else!
> > > jmc
> > > 
> > 
> > a little progress:
> > 
> > firstly i mis-diagnosed the issue. display.brightness=0 is not brighter
> > than the booted default! i'm not exactly sure how i reached that
> > conclusion.
> > 
> > but what i do see is values 97-100 severly dimming the screen. that
> > suggests a weird round up/bounds check (heh, i'm winging it here). i
> > noticed because i normally set max brightness in wsconsctl.conf, so it
> > dimmed my screen.
> > 
> > i narrowed it down to a commit between
> > 
> > 	Sun Aug 2 23:29:09 MDT 2026
> > and
> > 	Mon Aug 3 22:42:09 MDT 2026
> > 
> > (thanks nick for the archive reminder)
> > 
> > commits on those dates suggest the updates to drm. there are a lot, and
> > i'm blind. i'm ccing jsg in case he sees something obvious.
> > 
> > jmc
> 
> try this revert of 6.18.42's
> drm/amd/display: Fix backlight max_brightness to match exported range
> 

yes, the revert fixes it.

thanks,
jmc

> Index: sys/dev/pci/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> ===================================================================
> RCS file: /cvs/src/sys/dev/pci/drm/amd/display/amdgpu_dm/amdgpu_dm.c,v
> diff -u -p -r1.202 amdgpu_dm.c
> --- sys/dev/pci/drm/amd/display/amdgpu_dm/amdgpu_dm.c	3 Aug 2026 14:53:30 -0000	1.202
> +++ sys/dev/pci/drm/amd/display/amdgpu_dm/amdgpu_dm.c	10 Aug 2026 00:09:33 -0000
> @@ -5220,11 +5220,11 @@ amdgpu_dm_register_backlight_device(stru
>  	caps = &dm->backlight_caps[aconnector->bl_idx];
>  	if (get_brightness_range(caps, &min, &max)) {
>  		if (power_supply_is_system_supplied() > 0)
> -			props.brightness = DIV_ROUND_CLOSEST(max * caps->ac_level, 100);
> +			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->ac_level, 100);
>  		else
> -			props.brightness = DIV_ROUND_CLOSEST(max * caps->dc_level, 100);
> +			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->dc_level, 100);
>  		/* min is zero, so max needs to be adjusted */
> -		props.max_brightness = max;
> +		props.max_brightness = max - min;
>  		drm_dbg(drm, "Backlight caps: min: %d, max: %d, ac %d, dc %d\n", min, max,
>  			caps->ac_level, caps->dc_level);
>  	} else
>