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Subject: Re: Generic RGB LED support was Re: [PATCH 2/2] leds: lp5024: Add the
 LP5024/18 RGB LED driver
To:     Dan Murphy <dmurphy@ti.com>, Pavel Machek <pavel@ucw.cz>
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From:   Jacek Anaszewski <jacek.anaszewski@gmail.com>
Message-ID: <2c3425b0-70af-1f38-35ee-0af857bf0539@gmail.com>
Date:   Mon, 7 Jan 2019 21:59:42 +0100
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Dan,

On 1/7/19 8:36 PM, Dan Murphy wrote:
> Jacek
> 
> On 1/7/19 1:13 PM, Jacek Anaszewski wrote:
>> On 1/6/19 4:52 PM, Jacek Anaszewski wrote:
>>> Hi Pavel,
>>>
>>> On 1/5/19 11:12 PM, Pavel Machek wrote:
>>>> Hi!
>>>>
>>>>>> Grab yourself an RGB LED and play with it; you'll see what the
>>>>>> problems are. It is hard to explain colors over email...
>>>>>
>>>>> Video [0] gives some overview of lp5024 capabilities.
>>>>>
>>>>> I don't see any problems in exposing separate red,green,blue
>>>>> files and brightness for the devices with hardware support for
>>>>> that.
>>>>
>>>> Well, that's what we do today, as three separate LEDs, right?
>>>
>>> No. It doesn't allow for setting color intensity by having
>>> the color fixed beforehand. Below is relevant excerpt from
>>> the lp5024 documentation. This is not something that can be
>>> mapped to RGB color space, but rather to HSV/HSL, with the
>>> reservation that the hardware implementation uses PWM
>>> for setting color intensity.
>>>
>>> <quote>
>>> 8.3.1.2 Independent Intensity Control Per RGB LED Module
>>>
>>> When color is fixed, the independent intensity-control is used to
>>> achieve accurate and flexible dimming control for every RGB LED module.
>>>
>>> 8.3.1.2.1 Intensity-Control Register Configuration
>>>
>>> Every three consecutive output channels are assigned to their respective
>>> intensity-control register (LEDx_BRIGHTNESS). For example, OUT0, OUT1,
>>> and OUT2 are assigned to LED0_BRIGHTNESS, so it is recommended to
>>> connect the RGB LEDs in the sequence as shown in Table 1. The LP50xx
>>> device allows 256-step intensity control for each RGB LED module, which
>>> helps achieve a smooth dimming effect.
>>> </quote>
>>>
>>>> I don't have problem with that, either; other drivers already do
>>>> that. He's free to use existing same interface.
>>>>
>>>> But that is insufficient, as it does not allow simple stuff, such as
>>>> turning led "white".
>>>>
>>>> So... perhaps we should agree on requirements, first, and then we can
>>>> discuss solutions?
>>>>
>>>> Requirements for RGB LED interface:
>>>>
>>>> 1) Userspace should be able to set the white color
>>>>
>>>> 2) Userspace should be able to arbitrary color from well known list
>>>> and it should approximately match what would CRT, LCD or OLED monitor display
>>>
>>> The difference is that monitor display driver is pre-calibrated
>>> for given display by the manufacturer. With the LED controllers the
>>> manufacturer has no control over what LEDs will be connected to the
>>> iouts. Therefore it should be not surprising that colors produced
>>> by custom LEDs are not as user would expect when comparing to
>>> the RGB color displayed on the monitor display.
>>>
>>> TI provides "Various LED Ring Lighting Patterns Reference Design" [0]
>>> that show how to produce various patterns with use of the reference
>>> board with LED ring built using sixteen 19-337/R6GHBHC-A01/2T LEDs [1].
>>>
>>> Document [0] mentions also specific "Design considerations" in the
>>> chapter 2.2:
>>>
>>> <quote>
>>> Several considerations are taken into account for this particular design:
>>> ? LED map (ring) for meeting the requirement of popular human-machine interaction style.
>>> ? LED size, numbers and the diffuse design for meeting lighting pattern uniformity.
>>> ? Analog dimming in the difference ambient light lux without losing dimming resolution in lighting pattern.
>>>
>>> These considerations apply to most human-machine interaction end equipment with day and night vision
>>> designs in some way, but the designer must decide the particular considerations to take into account for a
>>> specific design.
>>> </quote>
>>>
>>> This renders your requirement 2) infeasible with use of custom LEDs
>>> any fixed algorithm, since the final effect will always heavily depend
>>
>> Typo here: s/any fixed/and fixed/
>>
>>> on the LED circuit design.
>>>
>>>>
>>>>  ???? 2a) LEDs probably slightly change color as they age. That's out of
>>>>  ???? scope, unless the variation is much greater than on monitors.
>>>>
>>>>  ???? 2b) Manufacturing differences cause small color variation. Again,
>>>>  ???? that's out of scope, unless the variation is much greater than on
>>>>  ???? monitors.
>>>>
>>>> Nice to have features:
>>>>
>>>> 3) Full range of available colors/intensities should be available to
>>>> userspace
>>>>
>>>> 4) Interface should work well with existing triggers
>>>>
>>>> 5) It would be nice if userland knew how many lumens are produced at
>>>> each wavelength for each setting (again, minus aging and manufacturing
>>>> variations).
>>>>
>>>> 6) Complexity of math in kernel should be low, and preferably it
>>>> should be integer or fixed point
>>>>
>>>> Problems:
>>>>
>>>> a) RGB LEDs are usually not balanced. Setting 100% PWM on
>>>> red/green/blue channels will result in nothing close to white
>>>> light. In fact, to get white light on N900, blue and green channel's
>>>> PWM needs to be set pretty low, as in 5%.
>>>>
>>>> b) LED class does not define any relation between "brightness" in
>>>> sysfs and ammount of light in lumens. Some drivers use close to linear
>>>> relation, some use exponential relation. Human eyes percieve logarithm
>>>> of lumens. RGB color model uses even more complex function.
>>>>
>>>> c) Except for white LEDs, LEDs are basically sources of single
>>>> wavelength of light, while CRTs and LCDs produce broader
>>>> spectrums.
>>>>
>>>> d) RG, RGBW and probably other LED combinations exist.
>>>>
>>>> e) Not all "red" LEDs will produce same wavelength. Similar
>>>> differences will exist for other colors.
>>>>
>>>> f) We have existing RGB LEDs represented as three separate
>>>> monochromatic LEDs in sysfs.
>>>
>>> One general question: do you have any solutions in store?
>>>
>>> [0] http://www.ti.com/lit/ug/tiduee6/tiduee6.pdf
>>> [1] http://www.everlight.com/file/ProductFile/19-337-R6GHBHC-A01-2T.pdf
>>>
>>
> 
> I just wanted to point out that there are 4 total devices right now that use the same mapping
> 
> LP5018, LP5024, LP5030 and the LP5036.
> 
> I can implement what ever we would like to I just need to know what to design against.

As you can see from the discussion in this thread it may take some
time to work out the interface satisfying everyone. I made some design
proposal, but Pavel had no warm word for it. It would be easier if
we had more opinions.

How do you feel about using brightness file for setting LEDn_BRIGHTNESS?

Does increasing LEDn_BRIGHTNESS value (i.e. color intensity) feel like
increasing color lightness (i.e. the pattern presented in the video [0]
starting from 1:22)?

> But keep in mind I still need to invest my time in the other TI-lmu patches on my list to complete.

Do what you deem most suitable for you. We are here only to help
merging the patches, but keeping in mind that kernel interface once
introduced must be preserved forever. Therefore we need to do our
best to make the best possible design decisions.

[0] https://www.youtube.com/watch?v=qdt-alh8i6E

-- 
Best regards,
Jacek Anaszewski