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

[Daniel Thompson <daniel.thompson@linaro.org>]

On Mon, Jun 10, 2019 at 01:52:33PM -0700, Matthias Kaehlcke wrote:
> 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.

This is a long standing flaw in the backlight interfaces. AFAIK generic
userspaces end up with a (flawed) heuristic.

Basically devices with a narrow range of choices can be assumed to be
logarithmic (since anything linear with a narrow range of choices is
junk anyway and the slider will never feel right).

On the other side though we can only really make a guess.

Systems are coming along that allow us to animate the change of
brightness (part of the reason for interpolated tables is to
permit smooth animation rather than because the user explicitly wants
to set the brightness to exactly 1117). These systems are often
logarithmic but with a wide range of values.


Daniel.