From patchwork Thu Aug 18 18:43:02 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Hans de Goede X-Patchwork-Id: 598528 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D2DE2C00140 for ; Thu, 18 Aug 2022 18:46:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345583AbiHRSqY (ORCPT ); Thu, 18 Aug 2022 14:46:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39836 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345589AbiHRSpY (ORCPT ); Thu, 18 Aug 2022 14:45:24 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C9F284E620 for ; Thu, 18 Aug 2022 11:45:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1660848321; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=3SH44BXwktH40KCcDZ9+emR32b1FaNxTyVmnyphrNZ0=; b=MnvHr0PUWdJ3+duIqOJE3NeubE6Z9JtZs2vuc5O1sNNTBsQkDoMYlwz4/lzsIbefmsyn1S MwcKrxX33zz2oV4gOWBolX/SiIPzGTG5tY3zF/bi+KN+doFrYrge5xYVG/U0uETCSUP8QA YA9ioRHrDcaDmATvtzDNcq8vzwafKmE= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-619-LFpjrYx6M1uDx4MF-Xar3w-1; Thu, 18 Aug 2022 14:45:20 -0400 X-MC-Unique: LFpjrYx6M1uDx4MF-Xar3w-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 490D285A581; Thu, 18 Aug 2022 18:45:18 +0000 (UTC) Received: from localhost.localdomain (unknown [10.39.192.20]) by smtp.corp.redhat.com (Postfix) with ESMTP id A84A7492C3B; Thu, 18 Aug 2022 18:45:14 +0000 (UTC) From: Hans de Goede To: Ben Skeggs , Karol Herbst , Lyude , Daniel Dadap , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , Alex Deucher , =?utf-8?q?Christian_K=C3=B6nig?= , Pan@vger.kernel.org, Xinhui , "Rafael J . Wysocki" , Mika Westerberg , Lukas Wunner , Mark Gross , Andy Shevchenko Cc: Hans de Goede , nouveau@lists.freedesktop.org, Daniel Vetter , David Airlie , intel-gfx , dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org, Len Brown , linux-acpi@vger.kernel.org, platform-driver-x86@vger.kernel.org Subject: [PATCH v3 31/31] drm/todo: Add entry about dealing with brightness control on devices with > 1 panel Date: Thu, 18 Aug 2022 20:43:02 +0200 Message-Id: <20220818184302.10051-32-hdegoede@redhat.com> In-Reply-To: <20220818184302.10051-1-hdegoede@redhat.com> References: <20220818184302.10051-1-hdegoede@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org Add an entry summarizing the discussion about dealing with brightness control on devices with more then 1 internal panel. The original discussion can be found here: https://lore.kernel.org/dri-devel/20220517152331.16217-1-hdegoede@redhat.com/ Signed-off-by: Hans de Goede --- Documentation/gpu/todo.rst | 68 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 68 insertions(+) diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst index 7634c27ac562..393d218e4a0c 100644 --- a/Documentation/gpu/todo.rst +++ b/Documentation/gpu/todo.rst @@ -679,6 +679,74 @@ Contact: Sam Ravnborg Level: Advanced +Brightness handling on devices with multiple internal panels +============================================================ + +On x86/ACPI devices there can be multiple backlight firmware interfaces: +(ACPI) video, vendor specific and others. As well as direct/native (PWM) +register programming by the KMS driver. + +To deal with this backlight drivers used on x86/ACPI call +acpi_video_get_backlight_type() which has heuristics (+quirks) to select +which backlight interface to use; and backlight drivers which do not match +the returned type will not register themselves, so that only one backlight +device gets registered (in a single GPU setup, see below). + +At the moment this more or less assumes that there will only +be 1 (internal) panel on a system. + +On systems with 2 panels this may be a problem, depending on +what interface acpi_video_get_backlight_type() selects: + +1. native: in this case the KMS driver is expected to know which backlight + device belongs to which output so everything should just work. +2. video: this does support controlling multiple backlights, but some work + will need to be done to get the output <-> backlight device mapping + +The above assumes both panels will require the same backlight interface type. +Things will break on systems with multiple panels where the 2 panels need +a different type of control. E.g. one panel needs ACPI video backlight control, +where as the other is using native backlight control. Currently in this case +only one of the 2 required backlight devices will get registered, based on +the acpi_video_get_backlight_type() return value. + +If this (theoretical) case ever shows up, then supporting this will need some +work. A possible solution here would be to pass a device and connector-name +to acpi_video_get_backlight_type() so that it can deal with this. + +Note in a way we already have a case where userspace sees 2 panels, +in dual GPU laptop setups with a mux. On those systems we may see +either 2 native backlight devices; or 2 native backlight devices. + +Userspace already has code to deal with this by detecting if the related +panel is active (iow which way the mux between the GPU and the panels +points) and then uses that backlight device. Userspace here very much +assumes a single panel though. It picks only 1 of the 2 backlight devices +and then only uses that one. + +Note that all userspace code (that I know off) is currently hardcoded +to assume a single panel. + +Before the recent changes to not register multiple (e.g. video + native) +/sys/class/backlight devices for a single panel (on a single GPU laptop), +userspace would see multiple backlight devices all controlling the same +backlight. + +To deal with this userspace had to always picks one preferred device under +/sys/class/backlight and will ignore the others. So to support brightness +control on multiple panels userspace will need to be updated too. + +There are plans to allow brightness control through the KMS API by adding +a "display brightness" property to drm_connector objects for panels. This +solves a number of issues with the /sys/class/backlight API, including not +being able to map a sysfs backlight device to a specific connector. Any +userspace changes to add support for brightness control on devices with +multiple panels really should build on top of this new KMS property. + +Contact: Hans de Goede + +Level: Advanced + Outside DRM ===========