linux-kernel - Re: [alsa-devel] [PATCH v4 3/3] ALSA: hda: Disabled unused audio controller for Dell platforms with Switchable Graphics

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Date:   Thu, 26 Apr 2018 15:52:08 +0800
From:   Kai Heng Feng <kai.heng.feng@...onical.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Pali Rohár <pali.rohar@...il.com>,
        Takashi Iwai <tiwai@...e.de>, mjg59@...f.ucam.org,
        dvhart@...radead.org, andy@...radead.org,
        Mario Limonciello <mario.limonciello@...l.com>,
        alsa-devel@...a-project.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org
Subject: Re: [alsa-devel] [PATCH v4 3/3] ALSA: hda: Disabled unused audio
 controller for Dell platforms with Switchable Graphics


> On Apr 25, 2018, at 5:13 AM, Lukas Wunner <lukas@...ner.de> wrote:
>
> On Mon, Apr 23, 2018 at 04:18:35PM +0800, Kai Heng Feng wrote:
>>> On Apr 23, 2018, at 4:08 PM, Pali RohÃ¡r <pali.rohar@...il.com> wrote:
>>> On Monday 23 April 2018 16:04:55 Kai Heng Feng wrote:
>>>>> On Apr 20, 2018, at 8:10 PM, Takashi Iwai <tiwai@...e.de> wrote:
>>>>> On Fri, 20 Apr 2018 11:44:32 +0200, Kai-Heng Feng wrote:
>>>>>> Now it's a typical discrete-only system. HDMI audio comes from AMD  
>>>>>> audio
>>>>>> controller, others from Intel audio controller.
>>>>>>
>>>>>> When SG is enabled, the unused AMD audio contoller still exposes its
>>>>>> sysfs, so userspace still opens the control file and stream. If
>>>>>> userspace tries to output sound through the stream, it hangs the  
>>>>>> system.
>>>>>
>>>>> Hm, could you give more information about how it hangs?
>>>>
>>>> Well, I should say "it hangs the userspace process" instead.
>>>>
>>>> $ speaker-test -t wav -c 2 -D hw:CARD=HDMI,DEV=3
>>>> ...and it just stopped. Can't Ctrl+C to break it.
>>>
>>> So userspace process cannot be killed at all? Then it is different bug
>>> in kernel and disabling pci device is just a workaround. Not a real fix.
>>>
>>> I would propose to find out what happen and why it cannot be killed
>>> (probably it stuck somewhere in kernel) and fix it properly.
>>
>> That's because the audio device got runtime suspended by the graphics.
>>
>> In this case, if we really want to use the the discrete audio, then we  
>> also
>> need to wake up the graphics.
>> The discrete audio is totally useless when SG is enabled, so my approach  
>> is
>> just to disable it.
>
> I don't quite follow, that should be fixed by commit 07f4f97d7b4b
> ("vga_switcheroo: Use device link for HDA controller") which landed
> in v4.17-rc1.

Looks like I hit a new bug: the discrete GFX and its audio controller never  
enters to D3.
The GFX can enter to D3 with my prosed patch.

>
> My understanding was that with SG enabled, the external DP/HDMI ports
> are muxed to the Intel GPU, so audio can only be streamed to external
> displays by the Intel HDA, not by the HDA integrated into the discrete
> AMD/Nvidia GPU.  Audio streamed to the latter would essentially end up
> in a blackhole.  And preventing the user from seeing such useless audio
> devices was the sole purpose of this commit.  Am I missing something?

Yes this is the intention.

Kai-Heng

>
> Thanks,
>
> Lukas