Date:	Thu, 23 Jun 2016 09:23:09 +0200
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Florian Vaussard <florian.vaussard@...g-vd.ch>
Cc:	Florian Vaussard <florian.vaussard@...il.com>,
	devicetree@...r.kernel.org, Richard Purdie <rpurdie@...ys.net>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] leds: ncp5623: Add device tree binding documentation

On 06/22/2016 04:25 PM, Florian Vaussard wrote:
> Hi Jacek,
>
> Le 22. 06. 16 à 10:51, Jacek Anaszewski a écrit :
>> Hi Florian,
>>
>> On 06/22/2016 08:08 AM, Florian Vaussard wrote:
>>> Hi Jacek,
>>>
>>> Le 21. 06. 16 à 17:28, Jacek Anaszewski a écrit :
>>>> Hi Florian,
>>>>
>>>> Thanks for the patch. I have two remarks below.
>>>>
>>>> On 06/21/2016 09:29 AM, Florian Vaussard wrote:
>>>>> Add device tree binding documentation for On Semiconductor NCP5623 I2C
>>>>> LED driver. The driver can independently control the PWM of the 3
>>>>> channels with 32 levels of intensity.
>>>>>
>>>>> The current delivered by the current source can be controlled using the
>>>>> led-max-microamp property. In order to control this value, it is also
>>>>> necessary to know the current on the Iref pin, hence the
>>>>> onnn,led-iref-microamp property. It is usually set using an external
>>>>> bias resistor, following Iref = Vref/Rbias with Vref=0.6V.
>>>>>
>>>>> Signed-off-by: Florian Vaussard <florian.vaussard@...g-vd.ch>
>>>>> ---
>>>>>     .../devicetree/bindings/leds/leds-ncp5623.txt      | 44
>>>>> ++++++++++++++++++++++
>>>>>     1 file changed, 44 insertions(+)
>>>>>     create mode 100644 Documentation/devicetree/bindings/leds/leds-ncp5623.txt
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/leds/leds-ncp5623.txt
>>>>> b/Documentation/devicetree/bindings/leds/leds-ncp5623.txt
>>>>> new file mode 100644
>>>>> index 0000000..0dc8345
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/leds/leds-ncp5623.txt
>>>>> @@ -0,0 +1,44 @@
>>>>> +* ON Semiconductor - NCP5623 3-Channel LED Driver
>>>>> +
>>>>> +The NCP5623 is a 3-channel I2C LED driver. The brightness of each
>>>>> +channel can be independently set using 32 levels. Each LED is represented
>>>>> +as a sub-node of the device.
>>>>> +
>>>>> +Required properties:
>>>>> +  - compatible: Should be "onnn,ncp5623"
>>>>> +  - reg: I2C slave address (fixed to 0x38)
>>>>> +  - #address-cells: must be 1
>>>>> +  - #size-cells: must be 0
>>>>> +  - onnn,led-iref-microamp: Current on the Iref pin in microampere
>>>>
>>>> I think that you don't need this property. Just provide the formula for
>>>> calculating led-max-microamp value, similarly as you're doing that in
>>>> the commit message.
>>>>
>>>
>>> I am not completely sure to understand your suggestion. So at the end, I have to
>>> compute the value of the register (let call it 'ILED') that I need to send to
>>> chip to configure the current source. The formula is:
>>>
>>> ILED = 31 - 2400*Iref/led-max-microamp
>>
>> led-max-microamp is the maximum current value for given LED.
>> According to the documentation it can be calculated as follows:
>>
>> ILEDmax = Iref * 2400 / (31 - n)
>>
>> Since this is global setting for all LEDs, then I'd always set n to 30,
>> and calculate max_brightness value for each LED separately, basing on
>> led-max-microamp property value. Effectively, I'm revoking my previous
>> statement about setting max_brightness to fixed level.
>>
>
> Ok your proposal simplifies a bit the handling. Thus ILEDmax of the current
> source would be always equal to Iref * 2400 and we use the PWM to limit the
> current inside the LED. The only downside of this approach is a reduced number
> of possible PWM steps, thus a limited number of RGB colors.

Yes, but by max_brightness being always 31, lowering led-max-microamp
results in decreasing the amount of current per brightness level.
Effectively, a human ability to notice perceived brightness level
change also decreases then.

In the approach I proposed this limitation is reflected in reduced
amount of available brightness levels.

> Regarding the DT binding, this would mean something like this:
>
> ncp5623@38 {
> 	#address-cells = <1>;
> 	#size-cells = <0>;
> 	compatible = "onnn,ncp5623";
> 	reg = <0x38>;
> 	led-max-microamp = <30000>;

Please drop it from here. It doesn't need to be configurable.
You can hard code this in the driver.

>
> 	ledr@0 {
> 		label = "ncp:power:red";
> 		reg = <0>;
> 		linux,default-trigger = "default-on";
> 		led-max-microamp = <5000>;

Is 5mA the maximum allowed current value for the LEDs on the board
you're using? Is brightness level change easily noticeable by max
current set to 5mA and max_brightness set to 31? It would be good
to empirically check this configuration.

> 	};
>
> 	ledb@1 {
> 		label = "ncp:power:blue";
> 		reg = <1>;
> 		led-max-microamp = <5000>;
> 	};
>
> 	ledg@2 {
> 		label = "ncp:power:green";
> 		reg = <2>;
> 		led-max-microamp = <5000>;
> 	};
> };
>
> The led-max-microamp property of the root node is used to infer Iref, and the
> led-max-microamp property inside each LED node is used to compute the maximum
> allowed PWM ratio (thus max_brightness).
>
> Would it be fine like this?
>
>> You can compare drivers/leds/leds-aat1290.c and its bindings, as it
>> uses similar approach.
>>
>
> Thanks for the pointer, interesting reading. In this case the flash-max-microamp
> property is implicitly used to get the value of Rset, and led-max-microamp is
> used to compute the flash/movie-mode ratio. Indeed similar but not exactly the
> same, as the NCP5623 allows a finer control on the current using one register to
> configure the current source and one register for the PWM.

Right, but it shows how led-max-microamp can be used to infer
max_brightness level. This is quite new DT property with not too many
users, because previously LED class drivers had been defining
max_brightness directly in a Device Tree. Nonetheless brightness level
was eventually considered not suitable unit for describing hardware
property.

-- 
Best regards,
Jacek Anaszewski

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