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Tue, 22 Sep 2026 11:41:34 +0000 Message-ID: <32f96551-358b-4874-8fd1-a685b02853e1@amd.com> Date: Tue, 22 Sep 2026 06:41:32 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 00/14] Add support for a DRM backlight capability Content-Language: en-US To: Javier Martinez Canillas Cc: dri-devel@lists.freedesktop.org, harry.wentland@amd.com, Simona Vetter , Alex Deucher , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Xaver Hugl , amd-gfx@lists.freedesktop.org, "open list:INTEL DRM DISPLAY FOR XE AND I915 DRIVERS" , "open list:INTEL DRM DISPLAY FOR XE AND I915 DRIVERS" , Hans de Goede References: <20260908044035.62093-1-mario.limonciello@amd.com> From: Mario Limonciello In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SN7PR18CA0013.namprd18.prod.outlook.com (2603:10b6:806:f3::31) To PH8PR12MB6914.namprd12.prod.outlook.com (2603:10b6:510:1cb::21) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR12MB6914:EE_|SA3PR12MB9130:EE_ X-MS-Office365-Filtering-Correlation-Id: 1013d3a1-1eb8-4581-1d26-08df189e7466 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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The main > issue for me is that if the sysfs interface is disabled, then I don't > understand the value of doing all the hops between the DRM and > backlight subsystems... The reason for all the hops is that users can switch between compositors that support this and don't. If you're in a compositor that supports it that compositor will want to affirm it's in control. If you're in a compositor without support then you should still have a way to change things, and that's what the sysfs interface exists for. > > IMO when a driver sets the DRIVER_CONNECTOR_LUMINANCE feature and the > client advertise the DRM_CLIENT_CAP_LUMINANCE capability, then the DRM > driver should be in full control of the brightness control and not go > through the backlight subsystem at all. OK but so let's say I start at 100% brightness. I open up Kwin, I change the luminance property to 0%. Let's pretend that backlight subsystem doesn't get updated. Then I log into Xorg + Xfce. The luminance property should be left at 0%, the brightness subsystem is 100%. The hardware would be left at 0%. I press the brightness up key (or call brightnessctl) and I can't change it because backlight subsystem is 100% already. So it can't be a clean rip and replace; you need to keep both active at least for a while. > > It would be better if the hardware specific logic to do the brightness > control is factored out and used by both the struct > backlight_ops.update_status and the atomic modesetting state check / > commit. > > -- > Best regards, > > Javier Martinez Canillas > Core Platforms > Red Hat