[Bug 101127] New: [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

            Bug ID: 101127
           Summary: [HSW, PWM] Brightness range not same as (correct)
                    inverted brightness range.
           Product: DRI
           Version: DRI git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: rafael.ristovski at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

A long time ago (I *think* around 4.8/4.9) I noticed that my usual PWM values
no longer corresponded to the physical amount of light produced by my screen.
A value of 1/937 which used to be ultra-dim was quite bright.
The cause of this was never found (Various PWM/brightness related commits at
that time reverted, to no avail).

Today I decided to try and invert the brightness via 'invert_brightness' in
sysfs and set the brightness to 936/937 - and there was the old ultra-dim
brightness I have seen before at 1/937.

What I think is happening is that the PWM range is shifted up, causing the
lowest PWM value to correspond to ~40+ of the old, correct range.
The highest PWM value seems to still clamp to 937, as that seems to be the hw
limit.

This causes the whole PWM range to be incorrect, not only disallowing low
brightness values, but also losing precision (Incorrect range is effectively
lower than 1-937).

tl;dr: Normal 1/937 much brighter than inverted 936/937. The latter being the
original correct brightness produced by 1/937.

Sysinfo:
Haswell Intel i7-4500U with HD 4400, eDP laptop display.
next-20170519 (Happens on ~4.9+)

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