Date:   Mon, 3 Apr 2017 22:38:58 +0200
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Pavel Machek <pavel@....cz>, Rob Herring <robh@...nel.org>,
        Richard Purdie <rpurdie@...ys.net>,
        linux-kernel@...r.kernel.org, linux-leds@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 1/2] leds: Add driver for Qualcomm LPG

On 04/03/2017 09:00 PM, Bjorn Andersson wrote:
> On Fri 31 Mar 02:28 PDT 2017, Jacek Anaszewski wrote:
> 
>> Hi Bjorn and Pavel,
>>
>> On 03/30/2017 09:43 AM, Pavel Machek wrote:
>>> Hi!
>>>
>>>>>> There is a binding for ti,lp55xx, but there's nothing I can reuse from
>>>>>> that binding...because it's completely different hardware.
>>>>>
>>>>> Agreed, if you drop the pattern stuff from the binding, at least for now. 
>>>>
>>>> I do not have a strong preference to expose these knobs in devicetree
>>>> and I do fear that finding some common "pattern" bindings that suits
>>>> everyone will be very difficult.
>>>>
>>>> So I'll drop them from the binding for now.
>>>
>>> Ok.
>>>
>>>>> If you want driver merged quickly, I believe the best way would be to
>>>>> leave out pattern support for now. We can merge the basic driver
>>>>> easily to 4.12.
>>>>>
>>>>
>>>> I'm not that much in a hurry and would rather see that we resolve any
>>>> outstanding issues with the implementation of the pattern handling.
>>>
>>> Ok, good.
>>>
>>>> But regardless of this we still have the problem that the typical
>>>> Qualcomm PMIC has 8 LPG-blocks and any triple could be driving a
>>>> RGB-LED. So we would have to create some sort of in-driver-wrapper
>>>> around any three instances exposing them as a single LED to the user.
>>>
>>> Yes, I believe we should do the wrapping. In N900 case, 
>>>
>>>> I rather expose the individual channels and make sure that when we
>>>> trigger a blink operation or enable a pattern (i.e. the two operations
>>>> that do require synchronization) we will perform that synchronization
>>>> under the hood.
>>>
>>> First, we need a way to tell userspace which LEDs are synchronized,
>>> because otherwise it will be confusing.
>>
>> There is one year old discussion [0] about the possible approaches
>> to RGB sub-LEDs synchronization problem and patterns in general.
>> My last message with API design proposal has been left unanswered.
>>
>> Probably we continue that discussion here.
>>
>> Generally Bjorn's drivers touch two yet to be addressed issues:
>> - RGB LED support
>> - Generic support for patterns
>>
>> It is likely that both issues can be solved by utilizing trigger
>> mechanism. The possible solution to the problem Bjorn tried to
>> address with /sys/class/leds/<led>/pattern comma separated list
>> could be a trigger with adjustable number of pattern intervals.
>>
>> The trigger once activated would create a directory with the
>> number of files corresponding to the number of requested intervals,
>> and then user could write an interval value by writing it to the
>> corresponding file. Somehow related approach has been implemented
>> for USB port LED trigger:
>>
>> 0f247626cbbf ('usb: core: Introduce a USB port LED trigger")
>>
>> In both RGB and pattern approaches we should assess
>> if it is acceptable to provide a pattern for trigger name,
>> e.g. blink-pattern-{num_intervals}.
>>
>> If so, then "echo transition-pattern-15" would create a directory
>> e.g. transition_intervals with files interval_0 to interval_14,
>> that could be adjusted by userspace.
>>
> 
> Having a RGB-trigger that proxy a accepts a userspace request of a
> brightness-tripple and sets the brightness on the individual associated
> LEDs sounds reasonable - but should probably be generalized to any
> number of LEDs.
> 
> A slightly related matter is the question on how to use a single LED for
> multiple trigger sources, e.g. how do I get a single LED to show
> activity of two MMCs?.

You would have to add a dedicated trigger, similar to usb port trigger,
I mentioned in the previous message.

> 
> For the patterns I don't know how a trigger for this would look like,
> how would setting the pattern of a trigger be propagated down to the
> hardware?

We'd need a new op and API similar to blink_set()/led_blink_set().

>>> Second, there are more issues than just patterns with the RGB
>>> LED. Most important is ability to set particular colors. You want to
>>> set the RGB LED to "white", but that does not mean you can set
>>> red=green=blue=1.0. You want color to look the same on LCD and on the
>>> LED, which means coefficients for white and some kind of function for
>>> brightness-to-PWM conversion.
>>
>> Shouldn't we leave that entirely to the userspace? Can we come up
>> with coefficients that will guarantee the same result on all existing
>> LCD devices?
>>
> 
> How about we just force user space perform the 3 writes and save us the
> cost of another trigger in that case? Configuring the brightness of 3
> LEDs is not board specific - and even with a RGB-interface we still need
> to specify which RGB-LED should be controlled.

This is what we have now, so we can live with it. Addition of a new
RGB trigger would be an improvement of the existing state.

-- 
Best regards,
Jacek Anaszewski

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