Re: [PATCH v5 13/13] video: backlight: mt6370: Add MediaTek MT6370 support

[Daniel Thompson <daniel.thompson@linaro.org>]

On Fri, Jul 15, 2022 at 02:38:45PM +0200, AngeloGioacchino Del Regno wrote:
> Il 15/07/22 13:26, ChiaEn Wu ha scritto:
> > From: ChiaEn Wu <chiaen_wu@richtek.com>
> >
> > MediaTek MT6370 is a SubPMIC consisting of a single cell battery charger
> > with ADC monitoring, RGB LEDs, dual channel flashlight, WLED backlight
> > driver, display bias voltage supply, one general purpose LDO, and the
> > USB Type-C & PD controller complies with the latest USB Type-C and PD
> > standards.
> >
> > This adds support for MediaTek MT6370 Backlight driver. It's commonly used
> > to drive the display WLED. There are 4 channels inside, and each channel
> > supports up to 30mA of current capability with 2048 current steps in
> > exponential or linear mapping curves.
> >
> > Signed-off-by: ChiaEn Wu <chiaen_wu@richtek.com>
>
> Hello ChiaEn,
>
> I propose to move this one to drivers/leds (or drivers/pwm) and, instead of
> registering a backlight device, register a PWM device.
>
> This way you will be able to reuse the generic backlight-pwm driver, as you'd
> be feeding the PWM device exposed by this driver to the generic one: this will
> most importantly make it easy to chain it with MTK_DISP_PWM (mtk-pwm-disp)
> with a devicetree that looks like...

Out of interest, does MT6370 have the same structure for backlights as the prior
systems using mtk-pwm-disp or was mtk-pwm-disp simply a normal(-ish) PWM
that relied on something on the board for all the constant current
driver hardware?


>
> 	pwmleds-disp {
> 		compatible = "pwm-leds";
>
> 		disp_led: disp-pwm {
> 			label = "backlight-pwm";
> 			pwms = <&pwm0 0 500000>;
> 			max-brightness = <1024>;
> 		};
> 	};
>
> 	backlight_lcd0: backlight {
> 		compatible = "led-backlight";
> 		leds = <&disp_led>, <&pmic_bl_led>;
> 		default-brightness-level = <300>;
> 	};

I think this proposal has to start with the devicetree bindings rather
than the driver. Instead I think the question is: does this proposal
result in DT bindings that better describe the underlying hardware?

This device has lots of backlight centric features (OCP, OVP, single
control with multiple outputs, exponential curves, etc) and its not
clear where they would fit into the "PWM" bindings.

Come to think of it I'm also a little worried also about the whole linear
versus exponential curve thing since I thought LED drivers were required
to use exponential curves.


Daniel.