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

[ChiaEn Wu]

On Mon, Jul 18, 2022 at 4:27 PM AngeloGioacchino Del Regno
<angelogioacchino.delregno at collabora.com> wrote:
>

<snip>

> >>
> >> 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?
> >
> >
>
> As per my understanding, mtk-pwm-disp is chained to other multimedia features of
> the display block of MediaTek SoCs, such as the AAL (adaptive ambient light),
> CABC (content adaptive backlight control) etc, other than being a normal(ish)
> PWM... that's the reason of my request.
>
> Moreover, in the end, this PMIC's backlight controller is just a "fancy" PWM
> controller, with OCP/OVP.
>
> >>
> >>      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?
> >
>
>  From how I understand it - yes: we have a fancy PWM (&pwm0) that we use
> to control display backlight (backlight-pwm)...
>
> Obviously, here we're not talking about OLEDs, but LCDs, where the backlight
> is made of multiple strings of WhiteLED (effectively, a "pwm-leds" controlled
> "led-backlight").
>
> Using PWM will also allow for a little more fine-grained board specific
> configuration, as I think that this PMIC (and/or variants of it) will be
> used in completely different form factors: I think that's going to be both
> smartphones and tablets/laptops... and I want to avoid vendor properties
> to configure the PWM part in a somehow different way.
>
> > 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.
> >
>
> For OCP and OVP, the only bindings that fit would be regulators, but that's
> not a regulator... and that's about it - I don't really have arguments for
> that.
>
> What I really want to see here is usage of "generic" drivers like led_bl
> and/or pwm_bl as to get some "standardization" around with all the benefits
> that this carries.
>
> > 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.
> >
>
> That probably depends on how the controller interprets the data, I guess,
> but I agree with you on this thought.

Hi Angelo,

MT6370 is just a SubPMIC, not an SoC, and is applied in cellular
telephones, tablet PCs, and portable instruments.
And the PWM mode of the MT6370 backlight driver is optional, and not
must be enabled.
>From our perspective, this MT6370 backlight driver is not the same as
mtk-pwm-disp related driver.
Thanks!

>
> Regards,
> Angelo

-- 
Best Regards,
ChiaEn Wu