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

[Kai Heng Feng]

> On Mar 8, 2018, at 7:30 PM, Lukas Wunner <lukas@xxxxxxxxx> 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@xxxxxxxxx> 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