Re: [PATCH 1/2] leds: ncp5623: Add device tree binding documentation

[Florian Vaussard]

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@xxxxxxxxxx>
>>>> ---
>>>>    .../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.

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>;

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

	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.

Regards,
Florian