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Subject: Re: [alsa-devel] [PATCH v2 3/3] ALSA: hda: Disabled unused audio
 controller for Dell platforms with Switchable Graphics
From:   Kai Heng Feng <kai.heng.feng@canonical.com>
In-Reply-To: <20180308113020.GA23073@wunner.de>
Date:   Fri, 9 Mar 2018 16:22:06 +0800
Cc:     mjg59@srcf.ucam.org, pali.rohar@gmail.com, dvhart@infradead.org,
        andy@infradead.org, Mario Limonciello <mario.limonciello@dell.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        platform-driver-x86@vger.kernel.org
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References: <20180308091023.9061-1-kai.heng.feng@canonical.com>
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 <20180308093848.GA24068@wunner.de>
 <1A1B9259-DC5B-4C1A-96BD-7FCACDBFFBB3@canonical.com>
 <20180308113020.GA23073@wunner.de>
To:     Lukas Wunner <lukas@wunner.de>
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> On Mar 8, 2018, at 7:30 PM, Lukas Wunner <lukas@wunner.de> wrote:
>
> On Thu, Mar 08, 2018 at 06:38:45PM +0800, Kai Heng Feng wrote:
>>> On Mar 8, 2018, at 5:38 PM, Lukas Wunner <lukas@wunner.de> wrote:
>>> On Thu, Mar 08, 2018 at 05:10:23PM +0800, Kai-Heng Feng wrote:
>>>> Some Dell platforms (Preicsion 7510/7710/7520/7720) have a BIOS option
>>>> "Switchable Graphics" (SG).
>>>>
>>>> When SG is enabled, we have:
>>>> 00:02.0 VGA compatible controller: Intel Corporation Device 591b (rev  
>>>> 04)
>>>> 00:1f.3 Audio device: Intel Corporation CM238 HD Audio Controller (rev  
>>>> 31)
>>>> 01:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
>>>> [AMD/ATI] Ellesmere [Polaris10]
>>>> 01:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Ellesmere
>>>> [Radeon RX 580]
>>>>
>>>> The Intel Audio outputs all the sound, including HDMI audio. The audio
>>>> controller comes with AMD graphics doesn't get used.
>>>>
>>>> When SG is disabled, we have:
>>>> 00:1f.3 Audio device: Intel Corporation CM238 HD Audio Controller (rev  
>>>> 31)
>>>> 01:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
>>>> [AMD/ATI] Ellesmere [Polaris10]
>>>> 01:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Ellesmere
>>>> [Radeon RX 580]
>>>>
>>>> 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 controller still exposes its
>>>> sysfs, so userspace still opens the control file and stream. If
>>>> userspace tries to output sound through the stream, it hangs when
>>>> runtime suspend kicks in:
>>>> [ 12.796265] snd_hda_intel 0000:01:00.1: Disabling via vga_switcheroo
>>>> [ 12.796367] snd_hda_intel 0000:01:00.1: Cannot lock devices!
>>>
>>> This should be fixed by the following series:
>>> https://lists.freedesktop.org/archives/dri-devel/2018-March/168012.html
>>>
>>> Please verify that by testing the series on the machine in question.
>>> I'm hoping to get those patches in for 4.17.  I suspect that your
>>> patch may not be necessary then.
>>
>> I no longer see the warning message with your patch. Thanks!
>
> Awesome, thanks for testing!
>
>
>>>> Since the discrete audio controller isn't useful when SG enabled, we
>>>> should just disable the device.
>>>
>>> I don't quite follow, when SG is enabled but hda_intel doesn't bind
>>> to the AMD audio device, how are you going to stream audio to
>>> external displays?  Are external DP/HDMI ports muxed to the integrated
>>> GPU when SG is enabled?
>>
>> Yes. It's a muxed port.
>> The can directly output via integrated Intel GPU when SG is enabled.
>> The discrete audio controller never gets used when SG is enabled.
>
> Okay, that's a crucial piece of information which I think should be
> made more explicit in the commit message and probably also in a code
> comment so that someone reading through hda_intel.c doesn't have to
> look in the git history to understand what's going on.
>
> I'd remove the portion of the commit message pertaining to runtime
> suspend and instead write something like:
>
>     If Switchable Graphics is enabled, external DP/HDMI ports are
>     muxed to the Intel GPU and HDA controller and therefore those
>     on the AMD HDA controller shouldn't be exposed to user space.

I'll add the explanation to both comment and commit log.
Thanks for all the info.

Kai-Heng

>
> Thanks,
>
> Lukas