[Bug 101127] [HSW, PWM] Brightness range not same as (correct) inverted brightness range.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101127

Jani Nikula <jani.nikula at intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |NEEDINFO

--- Comment #1 from Jani Nikula <jani.nikula at intel.com> ---
This patch [1] fixes the inverted range to be the same as the non-inverted
range. I know that's not what you were after, but that's a bug we have and you
(perhaps inadvertently) reported.

The VBT lists the minimum brightness to be used for the backlight. Historically
we ignored that, until it caused real issues on some board designs. A low
enough PWM duty cycle would pull down other signals on the eDP connector,
causing black screens, IIRC. The VBT minimum is there for the OEM to specify.
Obviously it also means you can't go as dim as you used to be able to go.

As to the brightness range reduction, it doesn't really matter. You'll only
have maybe 100-300 user distinguishable luminance levels anyway, depending on
the PWM modulation frequency and other factors. More important would be to add
support to account for the non-linearity of the luminance as a function of the
PWM duty cycle.

Please test the patch and see if that fixes the inconsistency between inverted
and non-inverted backlight brightness.

[1]
http://patchwork.freedesktop.org/patch/msgid/20170531083355.7898-1-jani.nikula@intel.com

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