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 :

quoted

Hi Florian,

Thanks for the patch. I have two remarks below.

On 06/21/2016 09:29 AM, Florian Vaussard wrote:

quoted

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 <redacted>
---
   .../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.

quoted

quoted

+  - 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