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

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Date:   Mon, 29 May 2017 09:41:17 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     Chen-Yu Tsai <wens@...e.org>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com
Subject: Re: [PATCH 3/3] ARM: sun8i: v3s: add backlight device node for
 Lichee Pi Zero LCD

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.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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