Re: [PATCHv3] arm64: dts: allwinner: a64: teres-i: enable backlight

[Maxime Ripard <maxime.ripard@bootlin.com>]

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Hi Harald,

On Fri, Feb 08, 2019 at 02:16:29PM +0000, Harald Geyer wrote:
> Enable pwm and add a pretty standard backlight node.
> 
> The regulator is always on, but we include it anyway, because it is
> required by the binding document.
> 
> Signed-off-by: Harald Geyer <harald@ccbib.org>
> ---
> The backlight node got dropped from the initial submission of the
> teres-i DT, because PWM support wasn't available in time, and I
> kind of forgot to resubmit once PWM was in. Sorry about that.
> 
> While testing this patch I noticed, that sometimes on boot the
> brightness is set to max_brightness instead of the default specified
> in DT. I couldn't reproduce it reliably, but it seems to be related
> to changing the pwm period. I guess the logic trying to detect
> the brightness set by the boot loader gets confused in some corner
> case if the pwm periods don't match. However unbinding and rebinding
> the device to the driver always made it go to the proper default
> brightness, so the problem is clearly not in the DT.
> 
> If the theory above is true, then it implies that the version of
> u-boot running on my laptop doesn't completely overwrite all pwm
> parameters. As this might be specific to my installation, I didn't
> mention the issue in the commit message.
> 
> Changes since v2:
>  * Drop all the other stuff that got merged a year ago.
>  * Rebased on current sunxi/for-next branch
>  * Add power-supply property (just to conform to the binding)
>  * Use slightly different brightness-levels
> 
> Actually I tried omitting brightness-levels completely, as it is now
> optional. However automatic calculation gives unreasonable results
> (depending on the exact value of pwm period). A report has been sent
> to the author of the code.
> 
> 
>  .../arm64/boot/dts/allwinner/sun50i-a64-teres-i.dts | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/arm64/boot/dts/allwinner/sun50i-a64-teres-i.dts b/arch/arm64/boot/dts/allwinner/sun50i-a64-teres-i.dts
> index 7b7b14ba58e6..2a78932d5d3b 100644
> --- a/arch/arm64/boot/dts/allwinner/sun50i-a64-teres-i.dts
> +++ b/arch/arm64/boot/dts/allwinner/sun50i-a64-teres-i.dts
> @@ -21,6 +21,15 @@
>  		serial0 = &uart0;
>  	};
>  
> +	backlight: backlight {
> +		compatible = "pwm-backlight";
> +		pwms = <&pwm 0 50000 0>;
> +		power-supply = <&reg_dcdc1>;
> +		brightness-levels = <0 5 10 15 20 30 40 55 70 85 100>;

The backlight perceived brightness should increase lineary with each
step, which means that if you have ten steps, each step should
increase the perceived brightness by 10%. The eye being what it is, a
exponential sequence is usually a much better approximation.

It looks good otherwise, thanks!
Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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