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

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Date:   Thu, 2 Jun 2022 23:31:29 +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.
>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.
>
> Best regards,
> Krzysztof