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Date:   Mon, 6 Jun 2022 08:20:22 +0200
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Subject: Re: [PATCH 1/2] dt-bindings: backlight: rt4831: Add the new property
 for ocp level selection
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To:     ChiYuan Huang <u0084500@gmail.com>
Cc:     Rob Herring <robh@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Lee Jones <lee.jones@linaro.org>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Jingoo Han <jingoohan1@gmail.com>, Pavel Machek <pavel@ucw.cz>,
        Helge Deller <deller@gmx.de>, cy_huang <cy_huang@richtek.com>,
        lucas_tsai@richtek.com,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        "open list:FRAMEBUFFER LAYER" <linux-fbdev@vger.kernel.org>,
        Linux LED Subsystem <linux-leds@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
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From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
In-Reply-To: <CADiBU3_REqNRb4UtT5OrVBKuhYL0NPAALisHTM76SrFCHUYMuA@mail.gmail.com>
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On 06/06/2022 03:39, ChiYuan Huang wrote:
> Krzysztof Kozlowski <krzysztof.kozlowski@linaro.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@linaro.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@linaro.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@richtek.com>
>>>>>>>>>
>>>>>>>>> Add the new property for ocp level selection.
>>>>>>>>>
>>>>>>>>> Signed-off-by: ChiYuan Huang <cy_huang@richtek.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?

This sounds differently so I propose to use this dedicated property with
the changes I asked for.


Best regards,
Krzysztof