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

[Jacek Anaszewski]

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.

You can compare drivers/leds/leds-aat1290.c and its bindings, as it
uses similar approach.

> 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,

OK, please define only max and min value then.

> 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


--
Best regards,
Jacek Anaszewski