From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthias Kaehlcke Subject: Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye. Date: Mon, 10 Jun 2019 13:52:33 -0700 Message-ID: <20190610205233.GB137143@google.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 Return-path: Content-Disposition: inline In-Reply-To: <20190608210226.GB2359@xo-6d-61-c0.localdomain> Sender: linux-kernel-owner@vger.kernel.org To: Pavel Machek Cc: Enric Balletbo i Serra , 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 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.