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

On 11/10/2018 06:19 PM, Vesa Jääskeläinen wrote:
> 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 <jacek.anaszewski@gmail.com>
>>>> Cc: Baolin Wang <baolin.wang@linaro.org>
>>>> Cc: Daniel Mack <daniel@zonque.org>
>>>> Cc: Dan Murphy <dmurphy@ti.com>
>>>> Cc: Linus Walleij <linus.walleij@linaro.org>
>>>> Cc: Oleh Kravchenko <oleg@kaa.org.ua>
>>>> Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
>>>> Cc: Simon Shields <simon@lineageos.org>
>>>> Cc: Xiaotong Lu <xiaotong.lu@spreadtrum.com>
>>>> ---
>>>>    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] https://lkml.org/lkml/2017/8/30/423
>
> 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
>
> or
>
> echo "21 223 242" > rgb
>
> or:
>
> 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?

Thank you for the description of your approach.

Predefined DT color definitions make an interesting option. Nonetheless,
your design assumes that for RGB LEDs max_brightness would have to be
always 1.

While it would solve the issue, it wouldn't allow to benefit from
the whole potential of RGB LEDs - think of having adjustable "color
brightness" (like in HSV color model).

How abut allowing for providing an array of color intensity/brightness
levels per each color? In the simplest case there could be a single
level per color, but it should be possible to provide up to 255 levels.

--
Best regards,
Jacek Anaszewski

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