linux-kernel - Re: [PATCH 00/22] Fallback to native backlight

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Date:   Mon, 24 Oct 2022 15:14:51 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Akihiko Odaki <akihiko.odaki@...nix.com>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jonathan Corbet <corbet@....net>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        "Lee, Chun-Yi" <jlee@...e.com>, Mark Gross <markgross@...nel.org>,
        Corentin Chary <corentin.chary@...il.com>,
        Cezary Jackiewicz <cezary.jackiewicz@...il.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Jonathan Woithe <jwoithe@...t42.net>,
        Ike Panhc <ike.pan@...onical.com>,
        Daniel Dadap <ddadap@...dia.com>,
        Kenneth Chan <kenneth.t.chan@...il.com>,
        Mattia Dongili <malattia@...ux.it>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        Azael Avalos <coproscefalo@...il.com>,
        Lee Jones <lee@...nel.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Helge Deller <deller@....de>,
        Robert Moore <robert.moore@...el.com>,
        dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        platform-driver-x86@...r.kernel.org,
        acpi4asus-user@...ts.sourceforge.net,
        ibm-acpi-devel@...ts.sourceforge.net, linux-fbdev@...r.kernel.org,
        devel@...ica.org
Subject: Re: [PATCH 00/22] Fallback to native backlight

On Monday 24 October 2022 21:58:57 Akihiko Odaki wrote:
> Regarding the second limitation, I don't even understand the difference
> between vendor and native. My guess is that a vendor backlight device uses
> vendor-specific ACPI interface, and a native one directly uses hardware
> registers. If my guess is correct, the difference between vendor and native
> does not imply that both of them are likely to exist at the same time. As
> the conclusion, there is no more motivation to try to de-duplicate the
> vendor/native combination than to try to de-duplicate combination of devices
> with a single type.

Hello! I just want to point one thing. On some Dell laptops there are
3 different ways (= 3 different APIs) how to control display backlight.
There is ACPI driver (uses ACPI), GPU/DRM driver (i915.ko; uses directly
HW) and platform vendor driver (dell-laptop.ko; uses vendor BIOS or
firmware API). Just every driver has different pre-calculated scaling
values. So sometimes user wants to choose different driver just because
it allows to set backlight level with "better" granularity. Registering
all 3 device drivers is bad as user does not want to see 3 display
panels and forcing registration of specific one without runtime option
is also bad (some of those drivers do not have to be suitable or has
worse granularity as other).