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Message-ID: <84d86b61-7694-2e22-3593-b11b3ce33cf8@redhat.com>
Date:   Mon, 24 Oct 2022 15:23:46 +0200
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Subject: Re: [PATCH 00/22] Fallback to native backlight
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To:     =?UTF-8?Q?Pali_Roh=c3=a1r?= <pali@kernel.org>,
        Akihiko Odaki <akihiko.odaki@daynix.com>
Cc:     David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Jonathan Corbet <corbet@lwn.net>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <lenb@kernel.org>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        "Lee, Chun-Yi" <jlee@suse.com>, Mark Gross <markgross@kernel.org>,
        Corentin Chary <corentin.chary@gmail.com>,
        Cezary Jackiewicz <cezary.jackiewicz@gmail.com>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        Jonathan Woithe <jwoithe@just42.net>,
        Ike Panhc <ike.pan@canonical.com>,
        Daniel Dadap <ddadap@nvidia.com>,
        Kenneth Chan <kenneth.t.chan@gmail.com>,
        Mattia Dongili <malattia@linux.it>,
        Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
        Azael Avalos <coproscefalo@gmail.com>,
        Lee Jones <lee@kernel.org>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Jingoo Han <jingoohan1@gmail.com>,
        Helge Deller <deller@gmx.de>,
        Robert Moore <robert.moore@intel.com>,
        dri-devel@lists.freedesktop.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org,
        intel-gfx@lists.freedesktop.org,
        platform-driver-x86@vger.kernel.org,
        acpi4asus-user@lists.sourceforge.net,
        ibm-acpi-devel@lists.sourceforge.net, linux-fbdev@vger.kernel.org,
        devel@acpica.org
References: <20221024113513.5205-1-akihiko.odaki@daynix.com>
 <746e5cc6-516f-8f69-9d4b-8fe237de8fd6@redhat.com>
 <edec5950-cec8-b647-ccb1-ba48f9b3bbb0@daynix.com>
 <20221024131451.lvkesdg3kvyvbi7n@pali>
From:   Hans de Goede <hdegoede@redhat.com>
In-Reply-To: <20221024131451.lvkesdg3kvyvbi7n@pali>
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Hi,

On 10/24/22 15:14, Pali Rohár wrote:
> 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).

Right and that is just one example of laptops which can register both
vendor + native backlight devices, which is why this whole series is
a bad idea.

Regards,

Hans


> 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).