linux-kernel - Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye.

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Message-ID: <20190610205233.GB137143@google.com>
Date:   Mon, 10 Jun 2019 13:52:33 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Doug Anderson <dianders@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Brian Norris <briannorris@...gle.com>,
        Guenter Roeck <groeck@...gle.com>,
        Lee Jones <lee.jones@...aro.org>,
        Alexandru Stan <amstan@...gle.com>, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED
 linearly to human eye.

Hi Pavel,

On Sat, Jun 08, 2019 at 11:02:26PM +0200, Pavel Machek wrote:
> Hi!
> 
> > > +	 * Note that this method is based on empirical testing on different
> > > +	 * devices with PWM of 8 and 16 bits of resolution.
> > > +	 */
> > > +	n = period;
> > > +	while (n) {
> > > +		counter += n % 2;
> > > +		n >>= 1;
> > > +	}
> > 
> > I don't quite follow the heuristics above. Are you sure the number of
> > PWM bits can be infered from the period? What if the period value (in
> > ns) doesn't directly correspond to a register value? And even if it
> > did, counting the number of set bits (the above loops is a
> > re-implementation of ffs()) doesn't really result in the dividers
> > mentioned in the comment. E.g. a period of 32768 ns (0x8000) results
> > in a divider of 1, i.e. 32768 brighness levels.
> > 
> > On veyron minnie the period is 1000000 ns, which results in 142858
> > levels (1000000 / 7)!
> > 
> > Not sure if there is a clean solution using heuristics, a DT property
> > specifying the number of levels could be an alternative. This could
> > also be useful to limit the number of (mostly) redundant levels, even
> > the intended max of 4096 seems pretty high.
> > 
> > Another (not directly related) observation is that on minnie the
> > actual brightness at a nominal 50% is close to 0 (duty cycle ~3%). I
> > haven't tested with other devices, but I wonder if it would make
> > sense to have an option to drop the bottom N% of levels, since the
> > near 0 brightness in the lower 50% probably isn't very useful in most
> > use cases, but maybe it looks different on other devices.
> 
> Eye percieves logarithm(duty cycle), mostly, and I find very low brightness
> levels quite useful when trying to use machine in dark room.

I realized that the brightness level display on Chrome OS (= my test
device) is non-linear, and it isn't actually the lower 50% of levels
that is near 0 brightness, but 'only' about 20%.

> But yes, specifying if brightness is linear or exponential would be quite
> useful.

Agreed, this could help userspace with displaying a reasonable
brightness level.