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From:   Alexandru Stan <amstan@chromium.org>
To:     Thierry Reding <thierry.reding@gmail.com>,
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        <u.kleine-koenig@pengutronix.de>, Lee Jones <lee.jones@linaro.org>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Jingoo Han <jingoohan1@gmail.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Heiko Stuebner <heiko@sntech.de>,
        Rob Herring <robh+dt@kernel.org>
Cc:     Matthias Kaehlcke <mka@chromium.org>,
        Douglas Anderson <dianders@chromium.org>,
        Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        Alexandru Stan <amstan@chromium.org>,
        devicetree@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-arm-kernel@lists.infradead.org, linux-fbdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org,
        linux-rockchip@lists.infradead.org
Subject: [PATCH 0/3] PWM backlight interpolation adjustments
Date:   Mon, 20 Jul 2020 21:25:19 -0700
Message-Id: <20200721042522.2403410-1-amstan@chromium.org>
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I was trying to adjust the brightness for a new chromebook:
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/2291209
Like a lot of panels, the low end needs to be cropped,
and now that we have the interpolation stuff I wanted to make use of it
and bake in even the curve.

I found the behavior a little unintuitive and non-linear. See patch 1
for a suggested fix for this.

Unfortunatelly a few veyron dts files were relying on this
(perhaps weird) behavior. Those devices also want a minimum brightness.
The issue is that they also want the 0% point for turning off the
display.
https://github.com/torvalds/linux/commit/6233269bce47bd450196a671ab28eb1ec5eb88d9#diff-e401ae20091bbfb311a062c464f4f47fL23

So the idea here is to change those dts files to only say <3 255> (patch
3), and add in a virtual 0% point at the bottom of the scale (patch 2).

We have to do this conditionally because it seems some devices like to
have the scale inverted:
  % git grep "brightness-levels\s*=\s*<\s*[1-9]"|cat
  arch/arm/boot/dts/tegra124-apalis-eval.dts:             brightness-levels = <255 231 223 207 191 159 127 0>;


Alexandru Stan (3):
  backlight: pwm_bl: Fix interpolation
  backlight: pwm_bl: Artificially add 0% during interpolation
  ARM: dts: rockchip: Remove 0 point in backlight

 arch/arm/boot/dts/rk3288-veyron-jaq.dts    |  2 +-
 arch/arm/boot/dts/rk3288-veyron-minnie.dts |  2 +-
 arch/arm/boot/dts/rk3288-veyron-tiger.dts  |  2 +-
 drivers/video/backlight/pwm_bl.c           | 78 +++++++++++-----------
 4 files changed, 42 insertions(+), 42 deletions(-)

-- 
2.27.0