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 13:39:28 -0700 Message-ID: <20190610203928.GA137143@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> <819ecbcd-18e3-0f6b-6121-67cb363df440@collabora.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Return-path: Content-Disposition: inline In-Reply-To: <819ecbcd-18e3-0f6b-6121-67cb363df440@collabora.com> Sender: linux-kernel-owner@vger.kernel.org To: Enric Balletbo i Serra Cc: Pavel Machek , 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 Enric On Mon, Jun 10, 2019 at 12:00:02PM +0200, Enric Balletbo i Serra wrote: > 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 Thanks for the confirmation that I didn't just miss some clever trick. I also think that some kind of DT property is needed, I'll try to come up with a reasonable name, keeping in mind that some devices might not want to use the entire range of levels.