linux-kernel - Re: [PATCH v2 3/3] ALSA: hda: Disabled unused audio controller for Dell platforms with Switchable Graphics

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Date:   Sat, 10 Mar 2018 11:40:33 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Kai Heng Feng <kai.heng.feng@...onical.com>, mjg59@...f.ucam.org,
        dvhart@...radead.org, andy@...radead.org,
        Mario Limonciello <mario.limonciello@...l.com>, tiwai@...e.com,
        platform-driver-x86@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        alsa-devel@...a-project.org
Subject: Re: [PATCH v2 3/3] ALSA: hda: Disabled unused audio controller for
 Dell platforms with Switchable Graphics

On Saturday 10 March 2018 07:50:39 Lukas Wunner wrote:
> Pretty much all modern Nvidia GPUs do have an integrated HDA
> controller, however it's possible to hide it by clearing a bit
> at offset 0x488 in the GPU's config space.  Some BIOSes hide
> the HDA if no external display is attached.
> 
> I could imagine that the BIOS of the Dell machines in question
> hides the HDA if Switchable Graphics is enabled.  If that is the
> case, be aware that there's an ongoing discussion to always expose
> the HDA controller because the behavior of some BIOSes to only
> expose the HDA when a display is attached causes massive problems
> with Linux' HDA driver:
> https://bugs.freedesktop.org/show_bug.cgi?id=75985
> 
> If we decide to always expose the HDA controller on Nvidia cards,
> you may need to also match for the Nvidia vendor ID here.

This is probably the example of reason why current solution based on
vendor id is fragile and could be broken in future... if such changes to
unhide devices are going to be implemented.

-- 
Pali Rohár
pali.rohar@...il.com

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