Date:   Mon, 31 Dec 2018 16:43:30 +0100
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Dan Murphy <dmurphy@...com>, robh+dt@...nel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-leds@...r.kernel.org
Subject: Re: [PATCH 2/2] leds: lp5024: Add the LP5024/18 RGB LED driver

On 12/30/18 6:35 PM, Pavel Machek wrote:
> On Sun 2018-12-30 18:09:35, Jacek Anaszewski wrote:
>> On 12/29/18 8:07 PM, Pavel Machek wrote:
>>> Hi!
>>>
>>>>>> With the "color" sysfs file it will make more sense to allow for user
>>>>>> defined color palettes.
>>>>>>
>>>>>
>>>>> I think defining these values in the device tree or acpi severely limits the devices
>>>>> capabilities.  Especially in development phases.  If the knobs were exposed then the user space
>>>>> can create new experiences.  The color definition should be an absolute color defined in the dt and
>>>>> either the framework or user space needs to mix these appropriately.  IMO user space should set the policy
>>>>> of the user experience and the dt/acpi needs to set the capabilities.
>>>>>
>>>>> I do like Pavels idea on defining the more standard binding pattern to "group" leds into a single group.
>>>>>
>>>>> Maybe the framework could take these groups and combine/group them into a single node with the groups colors.
>>>>
>>>> There is still HSV approach [0] in store. One problem with proposed
>>>> implementation is fixed algorithm of RGB <-> HSV color space conversion.
>>>> Maybe allowing for some board specific adjustments in DT would add
>>>> more flexibility.
>>>>
>>>> [0] https://lkml.org/lkml/2017/8/31/255
>>>
>>> Yes we could do HSV. Problem is that that we do not really have RGB
>>> available. We do have integers for red, green and blue, but they do
>>> not correspond to RGB colorspace.
>>
>> OK, so conversion from HSV to RGB would only increase the aberration.
>> So, let's stick to RGB - we've got to have some stable ground and this
>> is something that the hardware at least pretends to be compliant
>> with.
> 
> I'm not saying that we should stick to RGB. I'm just saying that
> problem is complex.
> 
> And no, hardware does not even pretend to be compliant with RGB color
> model ( https://en.wikipedia.org/wiki/RGB_color_model ). In
> particular, in RGB there is non-linear brightness curve.

Quotation from the wiki page you referred to:

"RGB is a device-dependent color model: different devices detect or
reproduce a given RGB value differently, since the color elements (such
as phosphors or dyes) and their response to the individual R, G, and B
levels vary from manufacturer to manufacturer, or even in the same
device over time. Thus an RGB value does not define the same color
across devices without some kind of color management"

This claim alone leaves much room for the manufacturers to pretend that
their devices are compliant with RGB model.

And the documentation of the hardware the discussed driver is for
also refers to RGB model in many places - e.g. see Table 1, page 15
in the document [0], where mapping of output triplets to an RGB module
is shown.

One thing that I missed is that the discussed hardware provides
LEDn_BRIGHTNESS registers for each RGB LED module, that can be
configured to set color intensity in linear or logarithmic fashion.

Actually this stands in contradiction with RGB model, since
change of "color intensity" means change of all RGB components.

We could use brightness file as for monochrome LEDs for that,
but we'd need to come up with consistent interface semantics
for all devices, also those which don't have corresponding
functionality. Probably this is the place where we could apply
some RGB<->HSV conversion, as color intensity feels something
more of HSV's saturation and value.

It would be good to hear from Dan how that looks in reality
in case of lp5024 device.

>> Our problem is how to set the color atomically. With HSV approach we
>> were to obviate the problem by mapping brightness file to the "V"
>> component of that color space, and write all H,S and V values to the
>> hardware only on write to brightness file.
> 
> I'm not sure how realistic the "atomic color" problem is. Computers
> are way faster than human vision.

With LEDn_BRIGHTNESS registers of lp5024 it seems that we need the
ability for grouping LEDs in triplets and be able to set their intensity
with a single write operation.

> I believe problem to start with is the "white" problem. Setting
> R=G=B=255 will _not_ result in anything close to white light on
> hardware I have.

RGBW LED controllers solve this problem. For the devices without
white/amber we cannot do more than the hardware allows for.

[0] http://www.ti.com/lit/ds/symlink/lp5024.pdf

-- 
Best regards,
Jacek Anaszewski

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