From: Jacek Anaszewski <jacek.anaszewski@gmail.com>
To: "Vesa Jääskeläinen" <dachaac@gmail.com>,
"Pavel Machek" <pavel@ucw.cz>, "Dan Murphy" <dmurphy@ti.com>
Cc: robh+dt@kernel.org, devicetree@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-leds@vger.kernel.org
Subject: Re: Generic RGB LED support was Re: [PATCH 2/2] leds: lp5024: Add the LP5024/18 RGB LED driver
Date: Sun, 13 Jan 2019 17:37:29 +0100 [thread overview]
Message-ID: <1e37a036-bf46-5717-69e3-7cf9a1917fee@gmail.com> (raw)
In-Reply-To: <1c53d379-64e5-4e72-5236-de8db77a5048@gmail.com>
Hi Vesa,
On 1/9/19 8:11 AM, Vesa Jääskeläinen wrote:
> Hi Pavel,
>
> On 09/01/2019 0.59, 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.
>>
>> So they have feature where they have independent controls for each
>> channel, then one common control per three channels. Other chips have
>> common control for all the LEDs, for example. We don't support that
>> currently; lets focus on the RGB thing first.
>>
>>>> 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.
>>
>> It is true that _chip_ manufacturer can not know what LEDs will be
>> connected. But _system_ manufacturer can and should know that, and
>> should tell be able to tell us in the dts.
>>
>>> This renders your requirement 2) infeasible with use of custom LEDs
>>> any fixed algorithm, since the final effect will always heavily depend
>>> on the LED circuit design.
>>
>> Depending on LED circuit design and actual LEDs connected is okay.. we
>> just need to get information from _system_ designer (not chip
>> designer), and pass it to a place where color is computed.
>>
>>>> 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.
>>>
>>> One general question: do you have any solutions in store?
>>
>> I played with LEDs on N900 over the weekend, yes.
>>
>> And getting reasonable colors seems to be possible, when a) and b) are
>> solved... a) seems to be more important than b).
>>
>> Now... this does not tell us how we should design kernel<->user
>> interface, but it should tell us that main goals - 1) and 2) are
>> possible.
>
> I was thinking about this calibration and color correctness thing and I
> am thinking a bit that it should be partly in kernel and partly in user
> space.
>
> For displays and printers there are defined icc-profiles that define how
> colors are mapped to particular device in cases when you want to have
> accurate color representation. In theory to get accurate LED output one
> could model LEDs with icc profile and then pick your color and convert
> it with icc profile to actual raw hardware values. Then this raw
> hardware value could be written from user space to kernel.
icc-profiles idea sounds interesting. We would have to adjust hsv->rgb
algorithm to take the profiles into account. I wonder how that would
work in practice.
> In kernel we could provide raw hardware value support and in case of PWM
> IC controlled LEDs we could provide curve mapping to linearize the
> behavior of values entered to enable use in cases where close enough is
> good enough.
>
> Eg. if you want to have "white" then you have your color space picker
> like sRGB(255,255,255). Then you would define icc profile it might
> translate to hardware raw values 253%, 230%, 225%.
> Then you would write this to kernel with:
>
> # set red, green and blue color elements
> echo "253 230 225" > color
>
> In this case however behavior of brightness node is challening in
> accuracy domain. Britghtness value 255 would of course provide 1:1
> mapping in this case.
>
> To go right to correct color and brightness at one go we could have
> optional brightness at the end of color value array:
>
> # set red, green and blue color element and brightness setting:
> echo "253 230 225 255" > color
>
> If you want to have fancier behavior for brightness in your system then
> we probably need to have configurable brightness model.
>
> - hardware, like in lp5024 case would map to hardware register (would be
> omitted if there is no such register)
> - onoff, would act like light switch ON/OFF eg. either configured value
> or all zeroes.
> - scaled, would multiply the color elements
> - hsl, would use hsl formula
> - and this can be extended later with some other models and allows us to
> start with with some models now.
>
> We could define this in devicetree and from sysfs a bit like with trigger:
>
> $ cat brightness_model
> [hardware] onoff scaled hsl
>
> $ echo "hsl" > brightness_model
>
> $ cat brightness_model
> hardware onoff scaled [hsl]
>
> Then we could have "color_names" or such sysfs entry to determine allow
> user space auto detection of led elements):
>
> $ cat color_names
> red green blue
>
> I suppose this model would provide flexibilty for multiple cases. Make
> it simple for most uses, allow accuracy with icc profiles for advanced
> users, would allow atomic color setting.
>
> I have updated (not yet in github) my tests to use color array model and
> color_names already and can play with brightness_model thing if this is
> something that is good path?
>
> What do you think?
--
Best regards,
Jacek Anaszewski
next prev parent reply other threads:[~2019-01-13 16:37 UTC|newest]
Thread overview: 106+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-19 16:26 [PATCH 0/2] LP5024/18 LED introduction Dan Murphy
2018-12-19 16:26 ` [PATCH 1/2] dt: bindings: lp5024: Introduce the lp5024 and lp5018 RGB driver Dan Murphy
2018-12-28 23:53 ` Rob Herring
2018-12-31 18:54 ` Dan Murphy
2019-01-08 20:33 ` Jacek Anaszewski
2019-01-08 20:53 ` Dan Murphy
2019-01-08 21:16 ` Jacek Anaszewski
2019-01-08 21:22 ` Dan Murphy
2019-01-09 20:12 ` Jacek Anaszewski
2019-01-09 21:12 ` Dan Murphy
2019-01-09 21:28 ` Jacek Anaszewski
2019-01-09 21:31 ` Dan Murphy
2019-01-10 18:44 ` Jacek Anaszewski
2019-01-10 19:22 ` Dan Murphy
2019-01-10 19:57 ` Jacek Anaszewski
2019-01-10 20:43 ` Dan Murphy
2019-01-10 22:03 ` Jacek Anaszewski
2019-01-10 23:51 ` Dan Murphy
2019-01-11 12:38 ` Dan Murphy
2019-01-11 21:52 ` Jacek Anaszewski
2019-01-12 17:09 ` Dan Murphy
2019-01-12 19:48 ` Jacek Anaszewski
2019-01-14 12:27 ` Dan Murphy
2019-01-14 20:11 ` Jacek Anaszewski
2019-01-14 20:14 ` Dan Murphy
2019-01-14 20:28 ` Jacek Anaszewski
2019-01-14 20:29 ` Dan Murphy
2018-12-19 16:26 ` [PATCH 2/2] leds: lp5024: Add the LP5024/18 RGB LED driver Dan Murphy
2018-12-19 19:04 ` Dan Murphy
2018-12-19 19:34 ` Pavel Machek
2018-12-19 19:41 ` Dan Murphy
2018-12-19 20:10 ` Pavel Machek
2018-12-19 20:41 ` Dan Murphy
2018-12-19 21:36 ` Jacek Anaszewski
2018-12-19 21:50 ` Dan Murphy
2018-12-20 12:40 ` Vesa Jääskeläinen
2018-12-20 13:56 ` Dan Murphy
2019-01-01 13:45 ` Vesa Jääskeläinen
2019-01-03 22:05 ` Jacek Anaszewski
2019-01-03 23:19 ` Vesa Jääskeläinen
2019-01-03 23:34 ` Pavel Machek
2019-01-04 19:49 ` Vesa Jääskeläinen
2019-01-04 20:43 ` Pavel Machek
2019-01-04 19:12 ` Jacek Anaszewski
2019-01-04 20:12 ` Pavel Machek
2019-01-04 21:37 ` Jacek Anaszewski
2019-01-04 22:07 ` Pavel Machek
2019-01-05 12:16 ` Jacek Anaszewski
2019-01-05 12:31 ` Pavel Machek
2019-01-05 13:16 ` Jacek Anaszewski
2019-01-05 22:12 ` Generic RGB LED support was " Pavel Machek
2019-01-06 15:52 ` Jacek Anaszewski
2019-01-07 19:13 ` Jacek Anaszewski
2019-01-07 19:36 ` Dan Murphy
2019-01-07 20:59 ` Jacek Anaszewski
2019-01-07 21:14 ` Dan Murphy
2019-01-08 21:18 ` Jacek Anaszewski
2019-01-08 21:25 ` Dan Murphy
2019-01-10 12:46 ` Dan Murphy
2019-01-10 19:23 ` Jacek Anaszewski
2019-01-10 19:58 ` Dan Murphy
2019-01-10 21:02 ` Jacek Anaszewski
2019-01-10 21:07 ` Dan Murphy
2019-01-08 22:59 ` Pavel Machek
2019-01-09 7:11 ` Vesa Jääskeläinen
2019-01-13 16:37 ` Jacek Anaszewski [this message]
2019-01-05 0:39 ` Vesa Jääskeläinen
2019-01-07 19:34 ` Dan Murphy
2019-01-09 6:20 ` Vesa Jääskeläinen
2019-01-07 21:13 ` Jacek Anaszewski
2019-01-07 21:15 ` Dan Murphy
2019-01-09 6:46 ` Vesa Jääskeläinen
2019-01-13 16:36 ` Jacek Anaszewski
2018-12-20 20:31 ` Jacek Anaszewski
2018-12-21 7:32 ` Jacek Anaszewski
2018-12-21 13:05 ` Dan Murphy
2018-12-29 18:28 ` Jacek Anaszewski
2018-12-29 19:07 ` Pavel Machek
2018-12-30 17:09 ` Jacek Anaszewski
2018-12-30 17:35 ` Pavel Machek
2018-12-31 15:43 ` Jacek Anaszewski
2018-12-31 15:47 ` Jacek Anaszewski
2018-12-31 19:15 ` Dan Murphy
2019-01-01 14:42 ` Jacek Anaszewski
2019-01-01 18:11 ` Dan Murphy
2019-01-01 22:06 ` Jacek Anaszewski
2018-12-31 16:28 ` Pavel Machek
2019-01-01 14:26 ` Jacek Anaszewski
2018-12-19 22:03 ` Pavel Machek
2018-12-19 22:08 ` Dan Murphy
2018-12-19 22:27 ` Pavel Machek
2018-12-20 1:31 ` Dan Murphy
2018-12-20 9:06 ` Pavel Machek
2018-12-20 14:03 ` Dan Murphy
2019-01-08 21:10 ` Jacek Anaszewski
2019-01-08 21:17 ` Dan Murphy
2019-07-22 20:59 ` Backlight in motorola Droid 4 Pavel Machek
2019-07-23 15:53 ` Dan Murphy
2019-07-24 8:22 ` Pavel Machek
2019-07-24 12:45 ` Pavel Machek
2019-07-24 15:10 ` Dan Murphy
2019-07-24 15:22 ` Dan Murphy
2019-07-26 14:38 ` Dan Murphy
2019-07-29 22:00 ` Pavel Machek
2019-07-30 18:21 ` Dan Murphy
2019-07-30 21:52 ` Pavel Machek
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