Hi, > Am 10.09.2016 um 10:20 schrieb Matthijs van Duin <matthijsvanduin@xxxxxxxxx>: > > On 10 September 2016 at 09:08, H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> 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-- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
