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

Pavel Machek
2019-06-08T21:02:26Z

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.

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