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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 753F4CCA485 for ; Tue, 12 Jul 2022 19:49:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235461AbiGLTt6 (ORCPT ); Tue, 12 Jul 2022 15:49:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57332 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233013AbiGLTs3 (ORCPT ); Tue, 12 Jul 2022 15:48:29 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id AF353BD69E for ; Tue, 12 Jul 2022 12:41:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1657654884; 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=BNa8+B8hQbJkomdXiiZ8CbDG6bgx/CFgdcmWbUfRtKk=; b=fsP61FhLH84mVXS8XXUfXy91SOgh3i52YEmjwFAQArqFnZ6ZIhs8nZa2Dg3oA6enQ6JKeW CdI+HwAzC3N9K8KQlLhUA7Tv+RH/uXJQwq1ZK2MhGfYGXponvF7TmYDLPsYO61IIw63oNH EretSm+X1v2+kzNdaQCk2NZooylvWE8= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-496-k_jVBFM1OdaYte3FnS7xnw-1; Tue, 12 Jul 2022 15:41:21 -0400 X-MC-Unique: k_jVBFM1OdaYte3FnS7xnw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 0DC3A380406A; Tue, 12 Jul 2022 19:41:20 +0000 (UTC) Received: from localhost.localdomain (unknown [10.39.192.30]) by smtp.corp.redhat.com (Postfix) with ESMTP id BF84440E80E0; Tue, 12 Jul 2022 19:41:15 +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=20K=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 v2 29/29] drm/todo: Add entry about dealing with brightness control on devices with > 1 panel Date: Tue, 12 Jul 2022 21:39:10 +0200 Message-Id: <20220712193910.439171-30-hdegoede@redhat.com> In-Reply-To: <20220712193910.439171-1-hdegoede@redhat.com> References: <20220712193910.439171-1-hdegoede@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: platform-driver-x86@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 10bfb50908d1..8548bab88062 100644 --- a/Documentation/gpu/todo.rst +++ b/Documentation/gpu/todo.rst @@ -704,6 +704,74 @@ 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 =========== -- 2.36.0