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

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Message-ID: <59143a9b-ab51-bd3b-e9db-93680415d205@heig-vd.ch>
Date:	Wed, 22 Jun 2016 08:08:21 +0200
From:	Florian Vaussard <florian.vaussard@...g-vd.ch>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>,
	Florian Vaussard <florian.vaussard@...il.com>
CC:	<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

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

I need two pieces of information that depends on the hardware setup; the current
on the Iref pin (which is setup by a bias resistor Iref = 0.6V/Rbias), and the
desired output current (usually limited due to the chosen LED or the required
brightness).

On the other hand, I could also simply ask people to put the right value of
'ILED' (between 0 - 31) directly in the device tree, but this is less user-friendly.

>> +  - led-max-microamp: Desired maximum current for each LED in microampere
>> +                      (maximum 30000uA)
> 
> Please add instead of (maximum ...):
> 
> Valid values: min - max, step by N (rounded {up|down})
> 
> E.g.
> 
> Valid values: 10000 - 30000, step by 1000 (rounded down)
> 

This is unfortunately not a linear relationship,

led-max-microamp = 2400*Iref/(31-ILED)

thus steps are not constant. This can be seen on figure 5 (p.9) of the datasheet
[1].

Thanks for your review,
Florian

[1] http://www.onsemi.com/pub/Collateral/NCP5623-D.PDF