[Bug 105051] Radeon sets max_brightness to -1, breaking GNOME backlight control on Macbook Pro mid-2015 11,5

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=105051

--- Comment #4 from Tim Sammut <tim.sammut@gmail.com> ---
(In reply to Alex Deucher from comment #1)
> Radeon hardware supports 256 levels of brightness (0-255).  The backlight
> integer property max_brightness is thus set to RADEON_MAX_BL_LEVEL (0xff). 
> It sounds like GNOME is misinterpreting the value.

Hi Alex, thanks. I don't think is a GNOME issue. This is via /sys and you can
see the max_brightness = -1 here:

$ for F in /sys/class/backlight/*/{type,max_brightness}; do echo $F: $(cat $F);
done | sort
/sys/class/backlight/gmux_backlight/max_brightness: -1
/sys/class/backlight/gmux_backlight/type: platform
/sys/class/backlight/radeon_bl0/max_brightness: 255
/sys/class/backlight/radeon_bl0/type: raw

Manually adjusting the brightness via `echo $level >
/sys/class/backlight/gmux_backlight/brightness` works, but GNOME control of
brightness and the on-screen display do not. 


> The radeon backlight code hasn't really changed since it was added so
> presumably something else did.  Can you bisect?  Since this is a mac,
> it's possible the GPU driver does not actually control the backlight.
> Everything is a one off on macs.

Yeah, but probably not for 3+ weeks... sorry.

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