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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 DD0A3C0650E for ; Mon, 1 Jul 2019 16:55:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A143B2146F for ; Mon, 1 Jul 2019 16:55:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="lRaikns7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729710AbfGAQz3 (ORCPT ); Mon, 1 Jul 2019 12:55:29 -0400 Received: from mail-pg1-f193.google.com ([209.85.215.193]:40239 "EHLO mail-pg1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729596AbfGAQz2 (ORCPT ); Mon, 1 Jul 2019 12:55:28 -0400 Received: by mail-pg1-f193.google.com with SMTP id w10so6319004pgj.7 for ; Mon, 01 Jul 2019 09:55:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=xDSI42+k7oV3eNOdMjBIpymPAXqrgf6nD9EQGoEDOSQ=; b=lRaikns7k0uCZd5C9Ln4pFCafvvy3X3J1z3IOKuS6jl6hcBL6bV2bF/9yEndlBwiut VSDmQY3n9WohNZyarR2lVtR/uCDI3dEyNmWB6NXRX16l+AlIqu+kYx5bpkmFUNpj4PmD 4WJtWiEUnvCoQuD2X2/ZiwMj36GFejsjvm2gw= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=xDSI42+k7oV3eNOdMjBIpymPAXqrgf6nD9EQGoEDOSQ=; b=aljc1tclpEZ2HvUiusLq5jTfgJi+o8C81DcIcJnS2YegYGsHA9gXd5GJm7thVVOD/f AmP2ScBGoFiwKAsHx7JMMPQyZC1lEsg2cRhuEr7UMO75xbcgiW+oomLb6xo+8G0+OQxu iPtaXxPp91Hzlam/Ulq/d+I6+6VBLXbbDz2N2Q2In0a6aDvYgz6jNrLI8+vjxINllQAA RpDUDZpYJ59udpW6EIGWmWGoeLm64JYSxCfzrwg8YtNOeXVyR1WjsiTSkfTfJ8ZVpVzo GBTRksL8QooGBP3qY2bN1tuFVy1Hdl1HyapwBMuWeA4RZ0n+JJYadCkCPK4gXlkCONoj 7XUQ== X-Gm-Message-State: APjAAAWtGutAqbKk8GnpP+lKORRVcqM6dCdq+LEaPjHYWJrNyRcKrFX1 b/Tmoqfy1KVaDzjMvwiSfGQk/w== X-Google-Smtp-Source: APXvYqxwphdbl0NMiWm88UC1X5R0XR+EGB+EX9ncZxUKCYgnZV3kX08POr1D29GjDrjDNRrLjmPDTQ== X-Received: by 2002:a17:90a:a404:: with SMTP id y4mr311822pjp.58.1562000127976; Mon, 01 Jul 2019 09:55:27 -0700 (PDT) Received: from localhost ([2620:15c:202:1:75a:3f6e:21d:9374]) by smtp.gmail.com with ESMTPSA id d2sm12367553pgo.0.2019.07.01.09.55.26 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 01 Jul 2019 09:55:27 -0700 (PDT) Date: Mon, 1 Jul 2019 09:55:23 -0700 From: Matthias Kaehlcke To: Daniel Thompson Cc: Pavel Machek , Thierry Reding , Lee Jones , Jingoo Han , Bartlomiej Zolnierkiewicz , linux-pwm@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Enric Balletbo i Serra , Douglas Anderson , Brian Norris , Jacek Anaszewski Subject: Re: [PATCH v2 2/4] backlight: Expose brightness curve type through sysfs Message-ID: <20190701165523.GD137143@google.com> References: <20190624203114.93277-1-mka@chromium.org> <20190624203114.93277-3-mka@chromium.org> <20190626145611.GA22348@xo-6d-61-c0.localdomain> <20190628083452.tlgcylwo34lxi4s6@holly.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190628083452.tlgcylwo34lxi4s6@holly.lan> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Fri, Jun 28, 2019 at 09:34:52AM +0100, Daniel Thompson wrote: > On Wed, Jun 26, 2019 at 04:56:11PM +0200, Pavel Machek wrote: > > Hi! > > > > > Export the type of the brightness curve via the new sysfs attribute > > > 'scale'. The value of the attribute may be a simple string like > > > 'linear' or 'non-linear', or a composite string similar to > > > 'compatible' strings of the device tree. A composite string consists > > > of different elements separated by commas, starting with the > > > most-detailed description and ending with the least-detailed one. An > > > example for a composite string is "cie-1931,perceptual,non-linear" > > > This brightness curve was generated with the CIE 1931 algorithm, it > > > is perceptually linear, but not actually linear in terms of the > > > emitted light. If userspace doesn't know about 'cie-1931' or > > > 'perceptual' it should at least be able to interpret the 'non-linear' > > > part. > > > > I'm not sure the comma-separated thing is a good idea. If it is, it should > > go to the Documentation, not to changelog. > > So I viewed the comma-separated thing as allow us to describe facts about > the scale used. > > In particular I suspect that some controllers will be non-linear *and* > non-perceptual and that some userspaces, particularly those that animate > backlight changes, may care enough about the difference to ask us to add > another fact to the set that describes that scale. > > Having said that I do share your concern that the comma-separated list > is overengineered and that all userspaces will end up implementing > something like: > > if (strstr("non-linear", scale) { > mode = PERCEPTUAL; > } else if (strstr("unknown", scale) { > mode = use_existing_hueristic(); > } else { > mode = LINEAR; > } I agree that this is not unlikely ... So let's just make it 'linear', 'non-linear' and 'unknown'? > > > +What: /sys/class/backlight//scale > > > +Date: June 2019 > > > +KernelVersion: 5.4 > > > +Contact: Daniel Thompson > > > +Description: > > > + Description of the scale of the brightness curve. The > > > + description consists of one or more elements separated by > > > + commas, from the most detailed to the least detailed > > > + description. > > > + > > > + Possible values are: > > > + > > > + unknown > > > + The scale of the brightness curve is unknown. > > > + > > > + linear > > > + The brightness changes linearly in terms of the emitted > > > + light, changes are perceived as non-linear by the human eye. > > > + > > > + non-linear > > > + The brightness changes non-linearly in terms of the emitted > > > + light, changes might be perceived as linear by the human eye. > > > > non-linear is not too useful as described. > > Agree. > > The idea is that allows a userspace with simple backlight needs to > simple map the brightness property directly to a slider using the > approach above without worrying about perceptual or (possible future) > logarithmic scales. Such an approach won't be perfect but it > probably won't feel horrible for the user either. > > Arguably the descriptions should move away from the raw factual > approach and describe what advise the kernel of offering the > userspace. ok, I'll change it in the next revision > > > + perceptual,non-linear > > > + The brightness changes non-linearly in terms of the emitted > > > + light, changes should be perceived as linear by the human eye. > > > + > > > + cie-1931,perceptual,non-linear > > > + The brightness curve was calculated with the CIE 1931 > > > + algorithm. Brightness changes non-linearly in terms of the > > > + emitted light, changes should be perceived as linear by the > > > + human eye. > > > > Is it useful to know difference between perceptual, and cie-1931? > > Depends how assertive the userspaces are! > > If they follow the "fix kernel bugs in the kernel" mantra rather than > implement workarounds and heuristics then I suspect it would not be used > much. > > > > Would it be useful to export absolute values in some well-known units? > > > > If I'm in dark room, I may want 100mW/m^2 of backlight... And it would > > be nice if I could set same backlight intensity on all my devices > > easily. > > I'm a little sceptical that we could calibrate an absolute scale on > enough devices for such a property to be useful. I think it would be > "unknown" on almost every system. I share your scepticism and would expect most devices to remain "unknown"