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

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Date:   Sun, 5 Jun 2022 18:11:20 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     ChiYuan Huang <u0084500@...il.com>
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

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?

> 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?

Best regards,
Krzysztof