Date:   Sun, 20 Jan 2019 16:32:25 +0100
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Dan Murphy <dmurphy@...com>, Pavel Machek <pavel@....cz>
Cc:     linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, dachaac@...il.com, robh+dt@...nel.org
Subject: Re: [PATCH v2 2/2] leds: lp50xx: Add the LP50XX family of the RGB LED
 driver

Dan,

On 1/18/19 2:45 PM, Dan Murphy wrote:
> Jacek
> 
> On 1/17/19 3:10 PM, Jacek Anaszewski wrote:
>> Hi Dan,
>>
>> On 1/16/19 7:41 PM, Dan Murphy wrote:
>>> Hello
>>>
>>> On 1/16/19 4:55 AM, Pavel Machek wrote:
>>>> Hi!
>>>>
>>>>> On 1/15/19 4:22 PM, Pavel Machek wrote:
>>>>>> Hi!
>>>>>>
>>>>>>>> +The 24-bit RGB value passed in follows the pattern 0xXXRRGGBB
>>>>>>>> +XX - Do not care ignored by the driver
>>>>>>>> +RR - is the 8 bit Red LED value
>>>>>>>> +GG - is the 8 bit Green LED value
>>>>>>>> +BB - is the 8 bit Blue LED value
>>>>>>>> +
>>>>>>>> +Example:
>>>>>>>> +LED module output 4 of the LP5024 will be a yellow color:
>>>>>>>> +echo 0xe6de00 > /sys/class/leds/lp5024\:led4_mod/color
>>>>>>>> +
>>>>>>>> +LED module output 4 of the LP5024 will be dimmed 50%:
>>>>>>>> +echo 0x80 > /sys/class/leds/lp5024\:led4_mod/brightness
>>>>>>>> +
>>>>>>>> +LED banked RGBs of the LP5036 will be a white color:
>>>>>>>> +echo 0xffffff > /sys/class/leds/lp5036\:led_banked/color
>>>>>>>
>>>>>>> This part with example cans remain in Documentation/leds if you
>>>>>>>> like.
>>>>>>
>>>>>> Does it actually work like that on hardware?
>>>>>
>>>>> What?
>>>>
>>>> If you do echo 0xffffff > /sys/class/leds/lp5036\:led_banked/color,
>>>> does it actually produce white? With all the different RGB modules
>>>> manufacturers can use with lp5024P?
>>>>
>>>> If you do echo 0xe6de00 > /sys/class/leds/lp5024\:led4_mod/color, does
>>>> it actually produce yellow, with all the different RGB modules
>>>> manufacturers can use with lp5024P?
>>>>
>>>
>>> I believe the answer to the general questions is no for any RGB cluster and driver out there.
>>> Because if you set the same values on each and every RGB device out there you will get varying shades of the color.
>>> But for this device yes the color does appear to be yellow to me versus what was displayed on my monitor by the HSL picker.
>>> But everyone interprets colors differently.
>>>
>>> If you write the same value for yellow or white on a droid 4 and the N900 do they produce the same color side by side?
>>> Most probably not.
>>>
>>> As you pointed out the PWM needs to be modified to obtain the correct white color to account for LED and other device constraints.
>>>
>>> But we need to take into account the light pipe.  Pools nowadays have RGB LED spot lights in them.  It can
>>> be set to white.  On my pool right off the lens the color has a purplish hue to it.  As the light is diffracted into
>>> the pool the color becomes white.  The pool is clear.  When I add chemicals to the pool and make it cloudy
>>> and turn on the lights the color off the lens is now white.  This is an example on a large scale but the issue
>>> scales down to the hand helds and smart home applications.
>>>
>>> If the cluster is piped through a flexible optic 0xffffff may produce the "white" you want on its output.
>>>
>>> So an expectation of certain color without proper piping based on a single RGB value may be a little unreasonable.
>>> There may need to be a way to attenuate the values based on the hardware aspect of the equation ie light pipe (or lack thereof) and LED vendor.
>>> So if we write 0xffffff to the RGB driver the driver could adjust the intensity of the individual LEDs based on the diffraction
>>> coefficients.
>>>
>>> I also think that is an unreasonable expectation here that writing a single value to any LED RGB driver would produce
>>> a "rest of the world" absolute color.  Maybe it can produce something similar but not identical.
>>> As you indicated in the requirements there is more involved here then just the LED and the values written.
>>> The colors should be close but may not be identical.
>>>
>>> A 10 year old N900 should not be considered the gold standard for color production due to advancements in LED,
>>> light pipe and LED driver technology.
>>> The single package RGB clusters on the board I am testing is about the size of a single RGB LED from 10 years ago.
>>>
>>> I agree that the interface developed should work on the device but the algorithm derived to obtain the color needs to have
>>> a hardware aspect to the calculation.
>>>
>>>>>> Is it supposed to support "normal" RGB colors as seen on monitors?
>>>>>
>>>>> Monitors are not an application for this part.
>>>>
>>>> You did not answer the question. When you talk about yellow, is it
>>>> same yellow the rest of world talks about?
>>>>
>>>
>>> See above.  It is close to what was on my monitor displayed.
>>>
>>>>>> Because 100% PWM on all channels does not result in white on hardware
>>>>>> I have.
>>>>>
>>>>> I don't know I am usually blinded by the light and have no diffuser over
>>>>> the LEDs to disperse the light so when I look I see all 3 colors.
>>>>
>>>> How can we have useful discussion about colors when you don't see the
>>>> colors?
>>>>
>>>> Place a piece of paper over the LEDs....
>>>>
>>>
>>> Good suggestion for a rough test.
>>>
>>>>>> But...
>>>>>>
>>>>>> I believe we should have a reasonable design before we do something
>>>>>> like this. There's no guarantee someone will not use lp50xx with just
>>>>>> the white LEDs for example. How will this work? Plus existing hardware
>>>>>> already uses three separate LEDs for RGB LED. Why not provide same
>>>>>> interface?
>>>>>
>>>>> Which existing hardware?  Are they using this part?
>>>>
>>>> Nokia N900. They are not using this part, but any interface we invent
>>>> should work there, too.
>>>>
>>>
>>> Yes a common interface would be nice with some sort of hardware tuning coefficient.
>>>
>>>>> <rant>
>>>>> Why are we delaying getting the RGB framework or HSV in?
>>>>> I would rather design against something you want instead of having
>>>>> everyone complain about every implementation I post.
>>>>> </rant>
>>>>
>>>> Because you insist on creating new kernel interfaces, when existing
>>>> interfaces work, and are doing that badly.
>>>>
>>>> Because your patches are of lower quality than is acceptable for linux
>>>> kernel.
>>>>
>>>> Because you don't seem to be reading the emails.
>>>>
>>>> I sent list of requirements for RGB led support. This does not meet
>>>> them.
>>>>
>>>
>>> Sigh.  You did not answer my question.
>>>
>>> Your requirements seem to be centered around monitors but that is only one application of the current
>>> RGB LED landscape.
>>>
>>> I am willing to work with you on the HSV and adapting the LP50xx part to this framework.
>>> Or any RGB framework for that matter.  I still don't agree with the kernel needing to declare colors
>>>    maybe color capabilities but not specific colors.
>>
>> Dan, if you have a bandwidth for LED RGB class implementation
>> then please go ahead. It would be good to compare colors produced
>> by software HSV->RGB algorithm to what can be achieved with
>> LEDn_BRIGHTNESS feature.
>>
>> The requirements for LED RGB class as I would see it:
>>
>> sysfs interface:
>>
>> brightness-model: space separated list of available options:
>> - rgb (default):
>>    - creates color file with "red green blue" decimal values
> 
> What about other colored LEDs?  Presenting RGB for an Amber LED does not seem right.
> Should the LED color come from the DT?

This is out of this discussion scope. Here we're discussing RGB LEDs.
I believe LEDn_BRIGHTNESS of lp50xx would also not work as intended with
colors other than RGB.

Best regards,
Jacek Anaszewski

> How to validate that the color is real?
> Or do we present a list of possible colors and validate that the color is appropriate?
> 
> I believe this could leverage the work you are doing on the LED label for color.
> 
>>    - creates brightness file
>>      a) for devices with hardware support for adjusting color
>>             intensity it maps to corresponding register
> 
> If we group LEDs as proposed we will have independent devices that give each LED a separate brightness control register.
> We would need to write to each brightness register here.
> 
>>          b) for the rest writing any value greater than 0 will result
>>             in setting all color registers to max
>> - hsv:
>>    - creates color file with "h s v" values - it shall
>>      use software HSV->RGB algorithm for setting color registers
>>
>> - any other custom color ranges defined in DT, but it can be covered
>>    later
>> - other options?
>>
>> Best regards,
>> Jacek Anaszewski
>>
>>
>>> It was agreed to continue forward with this particular implementation.
>>> At least thats what the email (I apparently did not read) stated.
>>>
>>> I need to fix the code to use the space separated value as pointed out and shown by Vesa.
>>> This will map nicely into this device with the color file as what I implemented is in theory
>>> they same code except for the space separated values.
>>>
>>>>> It is not a normal RGB driver.  The device collates the individual RGB
>>>>> clusters into a single brightness register and you can modify the intensity of the individual
>>>>> LEDs via other registers.  If brightness is 0 then the cluster is OFF regardless of the color
>>>>> set in the individual registers.
>>>>
>>>> I understand that. So just set cluster brightness to 255 and you have
>>>> normal RGB driver you can control with existing interfaces. You don't
>>>> have to use every feature your hardware has.
>>>>
>>>
>>> The brightness file is available and adjusts the brightness of the RGB cluster.
>>> I am not attempting to implement every feature the device has.  But I am attempting to use
>>> the basic features that are available and useful.
>>>
>>>> You did not answer the "what if someone uses this with all white LEDs"
>>>> question.
>>>>
>>>
>>> Are you asking what if someone places a white LED instead of a RGB on the hardware?
>>> Well then they need to go back and have a review of the data sheet and what they are trying to
>>> achieve.  That would be a misapplication of the LED driver itself and something software cannot fix.
>>>
>>> But if they do determine they want to control these white LEDs with this device
>>> then they can ignore the "color" file and control the cluster via the brightness file like we
>>> do today.  The color file will only change the intensity of the single output (assuming LED module mode)
>>> or the banked output.
>>>
>>> If a user wants to place a RGB cluster down on the hardware and have white as the consistent color
>>> well then that is fine as the RGB outputs are all set to 0xff and the intensity of the cluster is
>>> controlled by the brightness file.  If they cannot achieve the "white" with the default settings then
>>> on init they can set the color file once to obtain the "white" color and continue to use the brightness
>>> file to control the overall brightness of the cluster.
>>>
>>> It was determined in the email chain not to expose a brightness file per output as this device
>>> does not lend itself to that convention.
>>>
>>>> You know what? First, submit driver with similar functionality to
>>>> existing RGB drivers, using same interface existing drivers are
>>>> using. When that is accepted, we can talk about extending
>>>> kernel<->user interfaces.
>>>>
>>>
>>> I could do that but then there is no way for users to have any other color but "white" with this driver.
>>> That defeats the purpose of the device itself.
>>>
>>> This is why I would rather align the interfaces with what is being proposed so the interfaces won't change only
>>> the engine underneath will.
>>>
>>> I am not sure if you are aware of this or care but I found this recent blog on this effort:
>>> https://www.phoronix.com/scan.php?page=news_item&px=Linux-RGB-LED-Interface
>>> See some of the comments.
>>>
>>> Dan
>>>
>>>> Thanks,
>>>>                                      Pavel
>>>>
>>>
>>>
>>
>>
> 
> 

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