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=-6.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 5B698C31E49 for ; Thu, 13 Jun 2019 15:48:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3B01020B7C for ; Thu, 13 Jun 2019 15:48:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389624AbfFMPsz (ORCPT ); Thu, 13 Jun 2019 11:48:55 -0400 Received: from bhuna.collabora.co.uk ([46.235.227.227]:52428 "EHLO bhuna.collabora.co.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731710AbfFMJPD (ORCPT ); Thu, 13 Jun 2019 05:15:03 -0400 Received: from [127.0.0.1] (localhost [127.0.0.1]) (Authenticated sender: eballetbo) with ESMTPSA id 73D26260CB2 Subject: Re: [PATCH] backlight: pwm_bl: Fix heuristic to determine number of brightness levels To: Matthias Kaehlcke , Thierry Reding , Lee Jones , Daniel Thompson , Jingoo Han , Bartlomiej Zolnierkiewicz Cc: linux-pwm@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Douglas Anderson , Brian Norris , Pavel Machek , Jacek Anaszewski References: <20190612180003.161966-1-mka@chromium.org> From: Enric Balletbo i Serra Message-ID: Date: Thu, 13 Jun 2019 11:14:55 +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: <20190612180003.161966-1-mka@chromium.org> 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 12/6/19 20:00, Matthias Kaehlcke wrote: > With commit 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of > LED linearly to human eye") the number of set bits (aka hweight()) > in the PWM period is used in the heuristic to determine the number > of brightness levels, when the brightness table isn't specified in > the DT. The number of set bits doesn't provide a reliable clue about > the length of the period, instead change the heuristic to: > > nlevels = period / fls(period) > > Also limit the maximum number of brightness levels to 4096 to avoid > excessively large tables. > > With this the number of levels increases monotonically with the PWM > period, until the maximum of 4096 levels is reached: > > period (ns) # levels > > 100 16 > 500 62 > 1000 111 > 5000 416 > 10000 769 > 50000 3333 > 100000 4096 > > Fixes: 88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED linearly to human eye") > Signed-off-by: Matthias Kaehlcke Tested on Samsung Chromebook Plus (16-bit pwm) Tested-by: Enric Balletbo i Serra > --- > drivers/video/backlight/pwm_bl.c | 24 ++++++------------------ > 1 file changed, 6 insertions(+), 18 deletions(-) > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c > index fb45f866b923..0b7152fa24f7 100644 > --- a/drivers/video/backlight/pwm_bl.c > +++ b/drivers/video/backlight/pwm_bl.c > @@ -194,29 +194,17 @@ int pwm_backlight_brightness_default(struct device *dev, > struct platform_pwm_backlight_data *data, > unsigned int period) > { > - unsigned int counter = 0; > - unsigned int i, n; > + unsigned int i; > u64 retval; > > /* > - * Count the number of bits needed to represent the period number. The > - * number of bits is used to calculate the number of levels used for the > - * brightness-levels table, the purpose of this calculation is have a > - * pre-computed table with enough levels to get linear brightness > - * perception. The period is divided by the number of bits so for a > - * 8-bit PWM we have 255 / 8 = 32 brightness levels or for a 16-bit PWM > - * we have 65535 / 16 = 4096 brightness levels. > - * > - * Note that this method is based on empirical testing on different > - * devices with PWM of 8 and 16 bits of resolution. > + * Once we have 4096 levels there's little point going much higher... > + * neither interactive sliders nor animation benefits from having > + * more values in the table. > */ > - n = period; > - while (n) { > - counter += n % 2; > - n >>= 1; > - } > + data->max_brightness = > + min((int)DIV_ROUND_UP(period, fls(period)), 4096); > > - data->max_brightness = DIV_ROUND_UP(period, counter); > data->levels = devm_kcalloc(dev, data->max_brightness, > sizeof(*data->levels), GFP_KERNEL); > if (!data->levels) >