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=-2.3 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_MUTT 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 42474C31E46 for ; Wed, 12 Jun 2019 19:28:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 07FC721721 for ; Wed, 12 Jun 2019 19:28:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="jkCOi89f" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728779AbfFLT2W (ORCPT ); Wed, 12 Jun 2019 15:28:22 -0400 Received: from mail-pf1-f194.google.com ([209.85.210.194]:35473 "EHLO mail-pf1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728575AbfFLT0q (ORCPT ); Wed, 12 Jun 2019 15:26:46 -0400 Received: by mail-pf1-f194.google.com with SMTP id d126so10259109pfd.2 for ; Wed, 12 Jun 2019 12:26:45 -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=JGTJNpxtoSxf9cQjifYgXwRhQ7ZHC2LuIfFDHnbUz+Q=; b=jkCOi89f8z4k6wMOAeyYKznf4Y62pis19MeQNmtA7Jh53MHFibTLDJ9TYgKVn+f5kD fLUDfVGDMuu4aWcnxDzxbp07lL+Lz51Iy00qrEIJCzXdYG49TZOl7ZAvAbf9jiUmmh/V ih3U2U6lZeAoQ/pfzf5gNEqmoDoWlcBOwlo+E= 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=JGTJNpxtoSxf9cQjifYgXwRhQ7ZHC2LuIfFDHnbUz+Q=; b=QdF+c3YYecpf5ps+1otKW4uHtRIlmWsmtrbplz6OVmepczxmjfTLOvDtG0EFrIZAe4 VjQFn/EoUyaK0+nv0o1HdqrtodU38YT5sgS7hsu92HkVat4DleIpfV/Io3qxTf6WDTHD Our0BZKOCh7ooNh5c+2d4VlKWhvRzFtqCCFcejYiEHAL7DRaRemIFvFbYoogm5fL6LB0 BpAhMFO1KqWfiCoSV/Rpn/1ZhodumnQGaf+ck2A0nilQJqc0Qf/dybYDuQhyYIyefBQk OTQ6skTPPnVXuMmYE68FKQHeHP+hpLFa3uHBM6jDGQO/5achRowKmIobygXk8bMPD/HT G9mw== X-Gm-Message-State: APjAAAWmWiY2o7oT/F14OSedUOgLnOCBX7ld1C/MyHYY1tj3EVerynN7 GV21uEjezrDd2lN543j3+ZgHBQ== X-Google-Smtp-Source: APXvYqzXNaVC/oJkIo+F4M5Yj007p4G4RNdM/LCI3g6mVncptMolQVeByev0CceqM+5WvJuPiF17VA== X-Received: by 2002:a62:5306:: with SMTP id h6mr89297716pfb.29.1560367605628; Wed, 12 Jun 2019 12:26:45 -0700 (PDT) Received: from localhost ([2620:15c:202:1:75a:3f6e:21d:9374]) by smtp.gmail.com with ESMTPSA id e26sm326416pfn.94.2019.06.12.12.26.44 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 12 Jun 2019 12:26:44 -0700 (PDT) Date: Wed, 12 Jun 2019 12:26:42 -0700 From: Matthias Kaehlcke To: Daniel Thompson Cc: Brian Norris , Pavel Machek , Enric Balletbo i Serra , Doug Anderson , Rob Herring , Jingoo Han , Richard Purdie , Jacek Anaszewski , Guenter Roeck , Lee Jones , Alexandru Stan , linux-leds@vger.kernel.org, devicetree , Linux Kernel , kernel@collabora.com Subject: Re: [PATCH v3 3/4] backlight: pwm_bl: compute brightness of LED linearly to human eye. Message-ID: <20190612192642.GK137143@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> <20190610205233.GB137143@google.com> <20190611104913.egsbwcedshjdy3m5@holly.lan> <20190611223019.GH137143@google.com> <20190612110325.xdn3q2aod52oalge@holly.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190612110325.xdn3q2aod52oalge@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 Daniel, On Wed, Jun 12, 2019 at 12:03:25PM +0100, Daniel Thompson wrote: > On Tue, Jun 11, 2019 at 03:30:19PM -0700, Matthias Kaehlcke wrote: > > On Tue, Jun 11, 2019 at 09:55:30AM -0700, Brian Norris wrote: > > > On Tue, Jun 11, 2019 at 3:49 AM Daniel Thompson > > > wrote: > > > > This is a long standing flaw in the backlight interfaces. AFAIK generic > > > > userspaces end up with a (flawed) heuristic. > > > > > > Bingo! Would be nice if we could start to fix this long-standing flaw. > > > > Agreed! > > > > How could a fix look like, a sysfs attribute? Would a boolean value > > like 'logarithmic_scale' or 'linear_scale' be enough or could more > > granularity be needed? > > Certainly "linear" (this device will work more or less correctly if the > userspace applies perceptual curves). Not sure about logarithmic since > what is actually useful is something that is "perceptually linear" > (logarithmic is merely a way to approximate that). > > I do wonder about a compatible string like most-detailed to > least-detailed description. This for a PWM with the auto-generated > tables we'd see something like: > > cie-1991,perceptual,non-linear > > For something that is non-linear but we are not sure what its tables are > we can offer just "non-linear". Thanks for the feedback! It seems clear that we want a string for the added flexibility. I can work on a patch with the compatible string like description you suggested and we can discuss in the review if we want to go with that or prefer something else. > > The new attribute could be optional (it only exists if explicitly > > specified by the driver) or be set to a default based on a heuristic > > if not specified and be 'fixed' on a case by case basis. The latter > > might violate "don't break userspace" though, so I'm not sure it's a > > good idea. > > I think we should avoid any heuristic! There are several drivers and we > may not be able to work through all of them and make the correct > decision. Agreed > Instead one valid value for the sysfs should be "unknown" and this be > the default for drivers we have not analysed (this also makes it easy to > introduce change here). An "unknown" value sounds good, it allows userspace to just do what it did/would hace done before this attribute existed. > We should only set the property to something else for drivers that have > been reviewed. > > There could be a special case for pwm_bl.c in that I'm prepared to > assume that the hardware components downstream of the PWM have a > roughly linear response and that if the user provided tables that their > function is to provide a perceptually comfortable response. Unfortunately this isn't universally true :( At least several Chrome OS devices use a linear brightness scale and userspace does the transformation in the animated slider. A quick 'git grep -A10 brightness-levels arch' suggests that there are multiple other devices/platforms using a linear scale. We could treat devices with a predefined brightness table as "unknown", unless there is a (new optional) DT property that indicates the type of the scale. Cheers Matthias