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

[Kai Heng Feng]

> On Mar 10, 2018, at 2:50 PM, Lukas Wunner <lukas@xxxxxxxxx> wrote:
>
> On Fri, Mar 09, 2018 at 05:30:15PM +0800, Kai Heng Feng wrote:
> > > On Thursday 08 March 2018 17:10:23 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!
> > > >
> > > > Since the discrete audio controller isn't useful when SG enabled, we
> > > > should just disable the device.
> >
> > The platform does have a NVIDIA variant, but the discrete NVIDIA have a
> > audio controller, hence it doesn't have the issue.
>
> Sorry, I don't quite understand:  The AMD variant *also* has an audio
> controller, so what's the difference?  Or did you mean the Nvidia
> variant *doesn't* have an audio controller?

It does, but the audio controller only shows under SG disabled.

> 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.

This is how the Nvidia variant works:

When SG is enabled:
One audio controller, the integrated one.
All sounds comes from the integrated one, including DP/HDMI.

When SG is disabled:
Two audio controller.
The DP/HDMI sounds come from the Nvidia audio controller. Others are from  
the integrated audio controller.

> 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

The system in the bug doesn't have "Switchable Graphics".

Right now all sounds output are from integrated audio controller on SG  
enabled machines.

> If we decide to always expose the HDA controller on Nvidia cards,
> you may need to also match for the Nvidia vendor ID here.

Thanks for the info, I'll also put Nvidia vendor ID on next patch version.

> Thanks,
>
> Lukas