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

[Enric Balletbo i Serra <enric.balletbo@collabora.com>]

Hi Matthias,

On 8/6/19 23:02, 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.
>>

Right, I think that only works on the cases that we only have one pwm cell, and
looks like during my tests I did only tests on devices with one pwm cell :-(

And as you point the code is broken for other cases (pwm-cells > 1)

>> 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.
>>

Looking again looks like we _can not_ deduce the number of bits of a pwm, it is
not exposed at all, so I think we will need to end adding a property to specify
this. Something similar to what leds-pwm binding does, it has:

max-brightness : Maximum brightness possible for the LED


Enric

>> 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
>