linux-kernel - Re: [PATCH 3/3] ARM: sun8i: v3s: add backlight device node for Lichee Pi Zero LCD

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Message-ID: <CAGb2v66X2mAiEVnsj_aE8RV7DdRa_Pe41E-O-GOPe-+cjiw4PA@mail.gmail.com>
Date:   Mon, 29 May 2017 16:16:51 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     Chen-Yu Tsai <wens@...e.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>
Subject: Re: [PATCH 3/3] ARM: sun8i: v3s: add backlight device node for Lichee
 Pi Zero LCD

On Mon, May 29, 2017 at 3:41 PM, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> Hi,
>
> On Wed, May 24, 2017 at 08:13:20PM +0800, Icenowy Zheng wrote:
>> The 40-pin LCD connector on Lichee Pi Zero has backlight pins, which is
>> controlled by the PWM0 controller of the V3s SoC, and the controlling
>> part is on the board.
>>
>> Add the PWM and backlight device nodes in the device tree file, but
>> leave them disabled, as they can only be useful when the LCD is
>> attached. The LCD device tree overlay files can enable these controllers
>> and make use of them.
>>
>> Signed-off-by: Icenowy Zheng <icenowy@...c.io>
>> ---
>>  arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>>
>> diff --git a/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts b/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts
>> index 387fc2aa546d..e0721bf725d1 100644
>> --- a/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts
>> +++ b/arch/arm/boot/dts/sun8i-v3s-licheepi-zero.dts
>> @@ -52,6 +52,14 @@
>>               serial0 = &uart0;
>>       };
>>
>> +     backlight: backlight {
>> +             compatible = "pwm-backlight";
>> +             pwms = <&pwm 0 50000 0>;
>> +             brightness-levels = <0 10 20 30 40 50 60 70 80 90 100>;
>
> This is unlikely, the perceived brightness by the human eye is not
> linear, and is actually much closer to a logarithmic function, which
> means you should have use an exponential function here in order to
> appear that your brightness increase is linear.

I think the backlight is a functional part of the panel. It probably
belongs in the panel's overlay.

ChenYu