linux-kernel - Re: [PATCH 1/2] dt-bindings: backlight: rt4831: Add the new property for ocp level selection

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Date:   Mon, 6 Jun 2022 09:39:15 +0800
From:   ChiYuan Huang <u0084500@...il.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>, Pavel Machek <pavel@....cz>,
        Helge Deller <deller@....de>, cy_huang <cy_huang@...htek.com>,
        lucas_tsai@...htek.com,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:FRAMEBUFFER LAYER" <linux-fbdev@...r.kernel.org>,
        Linux LED Subsystem <linux-leds@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] dt-bindings: backlight: rt4831: Add the new property
 for ocp level selection

Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> 於 2022年6月6日 週一 上午12:11寫道：
>
> On 02/06/2022 17:31, ChiYuan Huang wrote:
> > Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> 於 2022年6月2日 週四 下午9:58寫道：
> >>
> >> On 02/06/2022 15:56, Rob Herring wrote:
> >>> On Thu, May 26, 2022 at 12:32:12PM +0200, Krzysztof Kozlowski wrote:
> >>>> On 26/05/2022 10:13, ChiYuan Huang wrote:
> >>>>> Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> 於 2022年5月26日 週四 下午4:06寫道：
> >>>>>>
> >>>>>> On 26/05/2022 05:16, cy_huang wrote:
> >>>>>>> From: ChiYuan Huang <cy_huang@...htek.com>
> >>>>>>>
> >>>>>>> Add the new property for ocp level selection.
> >>>>>>>
> >>>>>>> Signed-off-by: ChiYuan Huang <cy_huang@...htek.com>
> >>>>>>> ---
> >>>>>>>  .../bindings/leds/backlight/richtek,rt4831-backlight.yaml         | 8 ++++++++
> >>>>>>>  include/dt-bindings/leds/rt4831-backlight.h                       | 5 +++++
> >>>>>>>  2 files changed, 13 insertions(+)
> >>>>>>>
> >>>>>>> diff --git a/Documentation/devicetree/bindings/leds/backlight/richtek,rt4831-backlight.yaml b/Documentation/devicetree/bindings/leds/backlight/richtek,rt4831-backlight.yaml
> >>>>>>> index e0ac686..c1c59de 100644
> >>>>>>> --- a/Documentation/devicetree/bindings/leds/backlight/richtek,rt4831-backlight.yaml
> >>>>>>> +++ b/Documentation/devicetree/bindings/leds/backlight/richtek,rt4831-backlight.yaml
> >>>>>>> @@ -47,6 +47,14 @@ properties:
> >>>>>>>      minimum: 0
> >>>>>>>      maximum: 3
> >>>>>>>
> >>>>>>> +  richtek,bled-ocp-sel:
> >>>>>>
> >>>>>> Skip "sel" as it is a shortcut of selection. Name instead:
> >>>>>> "richtek,backlight-ocp"
> >>>>>>
> >>>>> OK, if so, do I need to rename all properties from 'bled' to 'backlight' ?
> >>>>> If  only this property is naming as 'backlight'. it may conflict with
> >>>>> the others like as "richtek,bled-ovp-sel".
> >>>>
> >>>> Ah, no, no need.
> >>>>
> >>>>>>
> >>>>>>> +    description: |
> >>>>>>> +      Backlight OCP level selection, currently support 0.9A/1.2A/1.5A/1.8A
> >>>>>>
> >>>>>> Could you explain here what is OCP (unfold the acronym)?
> >>>>> Yes. And the full name is 'over current protection'.
> >>>>
> >>>> Thanks and this leads to second thing - you encode register value
> >>>> instead of logical value. This must be a logical value in mA, so
> >>>> "richtek,bled-ocp-microamp".
> >>>
> >>> We already have common properties for setting current of LEDs. We should
> >>> use that here I think.
> >>
> >> It might not be exactly the same. We have "led-max-microamp" which is
> >> the maximum allowed current. I guess over-current protection level  is
> >> slightly higher (e.g. led-max-microamp + 1). IOW, led-max-microamp is
> >> something which still can be set and used by system/hardware. OCP should
> >> not.
> >>
> > Yap, you're right.
>
> So I am right or Rob?
>
As I know, both are incorrect.
> > From the modern backlight IC design, it uses the boost converter architecture.
> > This OCP level is to limit the inductor current when the internal MOS
> > switch turn on.
> > Details can refer to the below wiki link
> > https://en.wikipedia.org/wiki/Boost_converter
> >
> > And based on it, OVP is used to limit the inductor output voltage.
> > Each channel maximum current is based on the IC affordable limit.
> > It is more like as what you said 'led-max-microamp'.
> >
> > So boost voltage level may depend on the LED VF.
> > The different series of LED may cause different boost voltage.
> >
> > RT4831's OVP/OCP is not just the protection, more like as the limit.
>
> This suggests Rob is right, so let's use led-max-microamp property?
>
No, the meaning is different. 'led-max-microamp' always means the
channel output current.
It already can be adjusted by backlight brightness node.

For example
low voltage side (3.3~4.4V) to generate the boost voltage to 16~17V,
even 20V for BLED Vout.
This OCP is to limit the input current of low voltage side.

After the explanation, do you still think it's the same thing?
> Best regards,
> Krzysztof