From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0F607C282DD for ; Mon, 10 Jun 2019 10:00:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D78652085A for ; Mon, 10 Jun 2019 10:00:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388964AbfFJKAI (ORCPT ); Mon, 10 Jun 2019 06:00:08 -0400 Received: from bhuna.collabora.co.uk ([46.235.227.227]:56332 "EHLO bhuna.collabora.co.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388056AbfFJKAH (ORCPT ); Mon, 10 Jun 2019 06:00:07 -0400 Received: from [127.0.0.1] (localhost [127.0.0.1]) (Authenticated sender: eballetbo) with ESMTPSA id 7C75E279009 Subject: Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye. 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 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> From: Enric Balletbo i Serra Message-ID: <819ecbcd-18e3-0f6b-6121-67cb363df440@collabora.com> Date: Mon, 10 Jun 2019 12:00:02 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190608210226.GB2359@xo-6d-61-c0.localdomain> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@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 >