From mboxrd@z Thu Jan 1 00:00:00 1970 From: Enric Balletbo i Serra Subject: Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye. Date: Mon, 10 Jun 2019 12:00:02 +0200 Message-ID: <819ecbcd-18e3-0f6b-6121-67cb363df440@collabora.com> References: <20180208113032.27810-1-enric.balletbo@collabora.com> <20180208113032.27810-4-enric.balletbo@collabora.com> <20190607220947.GR40515@google.com> <20190608210226.GB2359@xo-6d-61-c0.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190608210226.GB2359@xo-6d-61-c0.localdomain> Content-Language: en-GB Sender: linux-kernel-owner@vger.kernel.org To: Pavel Machek , Matthias Kaehlcke Cc: Daniel Thompson , Doug Anderson , Rob Herring , Jingoo Han , Richard Purdie , Jacek Anaszewski , Brian Norris , Guenter Roeck , Lee Jones , Alexandru Stan , linux-leds@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, kernel@collabora.com List-Id: devicetree@vger.kernel.org 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 >