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Date:   Mon, 18 Sep 2017 17:00:47 +0100
From:   Daniel Thompson <>
To:     Enric Balletbo Serra <>,
        Doug Anderson <>
Cc:     Jingoo Han <>,
        Enric Balletbo i Serra <>,
        Lee Jones <>,
        Richard Purdie <>,
        Jacek Anaszewski <>,
        Pavel Machek <>, Rob Herring <>,
        Mark Rutland <>,
        Brian Norris <>,
        Guenter Roeck <>,,
        "" <>,
        "" <>,
        Alexandru M Stan <>
Subject: Re: Re: [RFC 0/2] backlight: pwm_bl: support linear brightness to
 human eye

On 14/09/17 11:46, Enric Balletbo Serra wrote:
>>>> So far in these discussions folks have been assuming that if we just
>>>> apply cie1931 to the PWM Duty Cycle then we're done and we have
>>>> perceived brightness in Lumens.  ...but I think that's not quite
>>>> right.  There are more factors.  Let's use the datasheet for a random
>>>> backlight driver, like RT8561A.  There appears to be a public
>>>> datasheet at
>>>> <>.
>>>> A) There may be a non-linear curve between PWM Duty Cycle and LED
>>>> Current (mA).  The particular curve is different based on mode
>>>> (Digital Ctrl vs. Analog Ctrl) and also PWM Frequency.  Sometimes this
>>>> curve is nearly linear for large parts of the curve but not the whole
>>>> curve.  Sometimes even though the curve is nearly linear there is an
>>>> offset (AKA 10% duty cycle could still produce nearly zero light
>>>> output).
>>>> B) There may be a non-linear curve between LED current and light
>>>> output in Watts (I think?).
>>>> C) The human perception model means there is a non-linear curve
>>>> between light output in Watts and human perceived brightness in
>>>> Lumens.
>>>> So A and B are hardware dependent and _do_ belong in the device tree
>>>> (IMHO).
>>> You forgot to model how to screen size and its maximum light output of the
>>> backlight impact pupil dilation ;-).
>> Silly me...  Oops, I also forgot to account for the absolute humidity
>> of the room.  Do you think we can require all backlights come with a
>> humidity sensor?
>>> Or... putting it another way, A and B are only relevant if they help us
>>> eliminate significant sources of error.
>> Right. your point is we can't model everything and we just need
>> to choose what's important.
>> I'll agree that "B" above might not be worth modelling (though I don't
>> know).  ...but I think we need to do _something_ about A.
>>  From the datasheet I point at looking at "Figure 8. LED Current vs.
>> ACTL PWM Dimming Duty Cycle", it seems like we at least need to do
>> something to account for the curve if we happen to be running at 30
>> kHz for whatever reason.  Specifically if we do no other work then any
>> duty cycle below 8% will result in no brightness.  Eyeballing the
>> graph 10% duty cycle looks to be about 2% current.
>> One option to solve this type of problem is to to specify a minimum
>> offset and assume things are linear after that offset.  That might
>> work, but it also might prevent you from accessing some of those nice
>> low brightness points.  Historically I have been frustrated when in
>> dark rooms that I couldn't set the brightness to be dim enough...
>> The whole piecewise linear concept is that maybe you'd specify the
>> curve (in terms of milliPercent) like this (values found by measuring
>> datasheet curve with a ruler):
>> <0 0>
>> <10000 1800> # 10% duty cycle gives 1.8% current
>> <12000 4300> # 12% duty cycle gives 4.3% current
>> <17000 10000> # 17% duty cycle gives 10% current
>> <93000 90000> # 93% duty cycle gives 90% current
>> <100000 100000> # 100% duty cycle gives 100% current
>>>> ...then the question is whether the device tree should specify the
>>>> curve so that the Watts scales linearly (and then the kernel adjust
>>>> for human perception) or so that Lumens scales linearly (which is
>>>> already adjusted for human perception).
>>>> Historically I believe the device tree has always wanted it so that
>>>> Lumens scales linearly.  So I guess the "we don't do anything" answer
>>>> is that the device tree should help account for for A + B + C.
>>> I would interpret the history slightly differently (although I'm not an
>>> authoritative historian here).
>>> There is a problem with the backlight interfaces (but entirely unrelated to
>>> Enric'c patch). The units the backlight users are not defined and varies
>>> from driver to driver.
> Based on this seems reasonable maintain current implementation to not
> break backward compability. Even, I think makes sense improve current
> implementation by adding somekind of piecewise linear concept to the
> brightness levels, similar to Doug's suggestion. So if we want, i.e,
> 256 levels or more, instead of specify the full table in the DT we can
> only specify some points in DT but the driver can expose to userspace
> more steps (how many?) between two brightness levels. Of course, this
> doesn't makes the live of the future users easier but I think will
> make the live of the current users of this interface more flexible
> (specially when you want lots of levels).

Ideally I'd like the driver to derive the number of steps based on the 
PWM resolution it discovers (I don't entirely agree that having a large 
portion of the slider map to no change is a good thing... we should be 
able to estimate the smallest useful step size).

Having said that, I'm open to suggestions about why we cannot make such 
an estimate.

> Then, to make the user live easier, there is the thing about human
> perception, we can move brightness-levels to be optional and fall to
> apply the human perception code if it's not specified. Here the thing
> and point of discussion is, if the cie1931 is the right algorithm to
> do the 'magic' in the driver. From what I investigated seems that is
> but I might be wrong.

It's certainly more correct than linear ;-) .

Actually I don't recall you commenting on the idea that we could ditch 
the fixed point code and simply have a default table built into the 
driver that can be used if there is no brightness-levels property 
(interpreted by the same piecewise linear code as everything else).

I suspect such a table could be fairly small.

> Seems reasonable apply both solutions? I can send a second RFC with
> both approaches.

Works for me.


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