[PATCH] arm64: dts: rockchip: Raise Pinebook Pro's panel backlight PWM frequency

[Nikola Radojevic]

Hello,
I have tested this on my Pinebook Pro and I can confirm that
everything seems to work alright.

Tested-by: Nikola Radojević <nikola at radojevic.rs>

On 24/08/04 11:10PM, Dragan Simic wrote:
> Increase the frequency of the PWM signal that drives the LED backlight of
> the Pinebook Pro's panel, from about 1.35 KHz (which equals to the PWM
> period of 740,740 ns), to exactly 8 kHz (which equals to the PWM period of
> 125,000 ns).  Using a higher PWM frequency for the panel backlight, which
> reduces the flicker, can only be beneficial to the end users' eyes.
> 
> On top of that, increasing the backlight PWM signal frequency reportedly
> eliminates the buzzing emitted from the Pinebook Pro's built-in speakers
> when certain backlight levels are set, which cause some weird interference
> with some of the components of the Pinebook Pro's audio chain.
> 
> The old value for the backlight PWM period, i.e. 740,740 ns, is pretty much
> an arbitrary value that was selected during the very early bring-up of the
> Pinebook Pro, only because that value seemed to minimize horizontal line
> distortion on the display, which resulted from the old X.org drivers causing
> screen tearing when dragging windows around.  That's no longer an issue, so
> there are no reasons to stick with the old PWM period value.
> 
> The lower and the upper backlight PWM frequency limits for the Pinebook Pro's
> panel, according to its datasheet, are 200 Hz and 10 kHz, respectively. [1]
> These changes still leave some headroom, which may have some positive effects
> on the lifetime expectancy of the panel's backlight LEDs.
> 
> [1] https://files.pine64.org/doc/datasheet/PinebookPro/NV140FHM-N49_Rev.P0_20160804_201710235838.pdf
> 
> Fixes: 5a65505a6988 ("arm64: dts: rockchip: Add initial support for Pinebook Pro")
> Cc: stable at vger.kernel.org
> Reported-by: Nikola Radojevic <nikola at radojevic.rs>
> Signed-off-by: Dragan Simic <dsimic at manjaro.org>
> ---
>  arch/arm64/boot/dts/rockchip/rk3399-pinebook-pro.dts | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/boot/dts/rockchip/rk3399-pinebook-pro.dts b/arch/arm64/boot/dts/rockchip/rk3399-pinebook-pro.dts
> index 294eb2de263d..b3f76cc2d6e1 100644
> --- a/arch/arm64/boot/dts/rockchip/rk3399-pinebook-pro.dts
> +++ b/arch/arm64/boot/dts/rockchip/rk3399-pinebook-pro.dts
> @@ -32,7 +32,7 @@ chosen {
>  	backlight: edp-backlight {
>  		compatible = "pwm-backlight";
>  		power-supply = <&vcc_12v>;
> -		pwms = <&pwm0 0 740740 0>;
> +		pwms = <&pwm0 0 125000 0>;
>  	};
>  
>  	bat: battery {