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SubjectRe: [PATCH 04/24] dt-bindings: leds: Add function and color properties
Hi Jacek,

On 09/11/2018 22.42, Jacek Anaszewski wrote:
> Hi Vesa,
> On 11/09/2018 09:32 AM, Vesa Jääskeläinen wrote:
>> On 07/11/2018 0.07, Jacek Anaszewski wrote:
>>> Introduce dedicated properties for conveying information about
>>> LED function and color. Mark old "label" property as deprecated.
>>> Signed-off-by: Jacek Anaszewski <>
>>> Cc: Baolin Wang <>
>>> Cc: Daniel Mack <>
>>> Cc: Dan Murphy <>
>>> Cc: Linus Walleij <>
>>> Cc: Oleh Kravchenko <>
>>> Cc: Sakari Ailus <>
>>> Cc: Simon Shields <>
>>> Cc: Xiaotong Lu <>
>>> ---
>>>   Documentation/devicetree/bindings/leds/common.txt | 52
>>> +++++++++++++++++++----
>>>   1 file changed, 44 insertions(+), 8 deletions(-)
>>> diff --git a/Documentation/devicetree/bindings/leds/common.txt
>>> b/Documentation/devicetree/bindings/leds/common.txt
>>> index aa13998..3efc826 100644
>>> --- a/Documentation/devicetree/bindings/leds/common.txt
>>> +++ b/Documentation/devicetree/bindings/leds/common.txt
>>> @@ -10,14 +10,20 @@ can influence the way of the LED device
>>> initialization, the LED components
>>>   have to be tightly coupled with the LED device binding. They are
>>> represented
>>>   by child nodes of the parent LED device binding.
>>>   +
>>>   Optional properties for child nodes:
>>>   - led-sources : List of device current outputs the LED is connected
>>> to. The
>>>           outputs are identified by the numbers that must be defined
>>>           in the LED device binding documentation.
>>> +- function: LED functon. Use one of the LED_FUNCTION_* prefixed
>>> definitions
>>> +        from the header include/dt-bindings/leds/functions.h.
>>> +        If there is no matching LED_FUNCTION available, add a new one.
>>> +- color : Color of the LED.
>> We have had for years out-of-tree patch for multi color gpio led driver
>> which extends this concept with multiple colors. Then in sysfs there has
>> been possibility to control the color and otherwise use blinking or
>> other features.
>> Our need is multi color status led of the device which includes
>> different kind of blinkings and colors on different situations.
>> Current in-tree gpio led driver just wasn't atomic enough and a bit
>> clumsy interface for handling this.
>> Now that this is being looked at could we come up with solution that we
>> could define multiple colors for one led in device tree and then we
>> could work on getting the driver upstreamed?
>> What we did was generally:
>> leds-multi {
>>     compatible = "gpio-multi-leds";
>>     status {
>>         gpios = <...>;
>>         linux,default-trigger = "none";
>>         deafult-state = "keep";
>>         color-red {
>>             pin-mask = <0x01>;
>>         };
>>         color-green {
>>             pin-mask = <0x02>;
>>         };
>>         color-orange {
>>             pin-mask = <0x03>;
>>         };
>>     };
>> };
> Device Tree node implementation doesn't actually say too much
> about the sysfs interface you came up with, which is most vital
> in this case.

In our case it is very simple. There is "color" named sysfs node under
gpio instance.

It creates own led instance for each children in this case it is named
as "status" and then uses properties under it to define operation.

Currently we do have fixed list of color names in driver to require
"standardized" names but that could be easily changed to be dynamic.

Driver registers all GPIOs defined in "gpios" under the instance.

So one can say:

echo "orange" > color

This setting goes to the driver, it figures out logical name "orange"
and the configure all listed GPIO's to its "pin-mask" state. Polarity of
the GPIO's is configured in GPIO definition so this is more or less turn
this particular pin to activate state.

So in this case it changes led's color to orange and still lets one to
use standard led triggers. Eg. set periodic blinking or so.

If one cat's "color" it lists all available colors for user and shows
which one is active.

When brightness is configured as zero the all registered go to off state
and when non-zero then it goes to state what ever is configured with
"color" sysfs entry.

> Support for RGB LEDs, or other variations of LED synchronization
> have been approached several times, but without satisfying result
> so far.
> Generally the problem boils down to the issue of how triggers
> should handle multiple synchronized LED class devices in a backwards
> compatible way with monochrome LEDs.
> At some point the HSV [0] approach was proposed, but there was a problem
> with getting uniform colors across devices. Especially white.
> Certainly a calibration mechanism would be required.
> [0]

We do not usually use PWM controlled leds so our calibration has been HW
engineer tuning the resistors for the leds to get acceptable color for
different colors variations.

If one wants to have fixed colors for such device then one could use
this similar method to define them or/and free form interface to enter
RGB values there. Thou with PWM RGB leds you probably want to have more
animation there which probably would require some additional support code.

One way to do atomic PWM RGB color change with sysfs could be:

echo "21 223 242" > color


echo "21 223 242" > rgb


echo "r:21 g:232 b:242" > color (or something similar)

and if there is know registered name then just write it to "color" which
would pick registered color rgb values to led instances rgb value.

Now for PWM RGB led one could use "brightness" and "rgb" value to
calculate actual color with some color space formula (like hsv in [0]).

Doing white with RGB LED is a bit hard so usually you want to get RGBW
LED (or RGBAW LED) if "real" white is something that is needed. This
could then be "rgbw" entry and "color" to pick from fixed set.

These presets in device tree for "color" could be considered one way of
doing calibration for particular hardware.

So in device tree for RGB PWM led it could be like:

color-orange {
rgb = <249 197 9>

color-warm-white {
rgb = <255 253 249>

How would that sound like?

Vesa Jääskeläinen

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