Re: [PATCH 1/2] leds: ncp5623: Add device tree binding documentation
From: Jacek Anaszewski
Date: Thu Jun 23 2016 - 03:23:20 EST
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@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.
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