linux-kernel - Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz

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Date:   Sat, 10 Sep 2016 11:10:17 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Matthijs van Duin <matthijsvanduin@...il.com>
Cc:     David Rivshin <drivshin@...worx.com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Marek Belisko <marek@...delico.com>,
        "linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>,
        Neil Armstrong <narmstrong@...libre.com>
Subject: Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz

Hi,

> Am 10.09.2016 um 10:20 schrieb Matthijs van Duin <matthijsvanduin@...il.com>:
> 
> On 10 September 2016 at 09:08, H. Nikolaus Schaller <hns@...delico.com> wrote:
>> Reducing the PWM frequency is good by itself since it should not be unnecessarily
>> fast and helps to make the PWM to "average current" translation more linear.
>> 
>> The non-linear effect is that the PWM controlled DC/DC converter reacts almost
>> immediately to a 1->0 control transition but needs some time (ca. 0.5ms) to recover
>> on a 0->1 transition.
> 
> DT already allows for compensation of many non-linearities by
> specifying the duty cycle of each brightness increment.  Though, as
> you observed, there's one limitation it cannot fix here:
> 
>> If we just fix the PWM generator to output a steady 1 signal at 100%, we have a
>> very significant change if we switch to 99%, depending on PWM frequency.
> 
> Specifically the next-to-brightest step (assuming 0.5ms off-time) would be:
> 75%  @ 500 Hz
> 90%  @ 200 Hz
> 95%  @ 100 Hz
> 96%  @  83 Hz

Yes.

> 
> Note that perceptually the distance to 100% might be smaller due to
> non-linear response of the eye. That's my experience with pwm
> controlled leds anyway, which may or may not apply to backlights

basically it does. Eye is basically logarithmic - but has several auto-exposure
and auto-iris mechanisms... So perceived brightness is a very complex topic.
It might even depend on the color and contrast of the image presented. This
is something we can ever fix by DT...

> (though with my laptop's backlight I never really have use for the
> distinct steps at the brightest end while those at the darkest end
> seem disproportionally large).
> 
>> This effect becomes smaller if the PWM frequency is reduced and 83Hz seems more
>> reasonable (although still a little arbitrary) than the current value.
> 
> While 500Hz is perhaps a bit high, 83Hz actually seems very low to me.

Why? The eye can't even see flicker @ 50 Hz.

And, there is a capacitor that averages the voltage applied, hence it is
low pass filtered. But the capacitor can't compensate for the startup delay
of the DC/DC converter.

And, I have tested that on the device targeted by this DTS... No visible
issue (except that maximum brightness decreases if too high).

> Searching a bit around yielded 175 Hz as common frequency for CCFL
> backlights and higher for LED backlights (source:
> http://www.tftcentral.co.uk/articles/pulse_width_modulation.htm).

> 
> (I may be reacting a bit twitchy here due to having encountered dimmed
> LED lighting that was flickering obnoxiously for me while noone else
> noticed this.)
> 
> Matthijs

But with the patch submitted, I just want to give the dts of a single
device I have even designed a more reasonable value than in current
linux/master and don't really want to make it a fundamental discussion...

BR,
Nikolaus