Re: [Nouveau] [PATCH v2] ALSA: hda: Continue to probe when codec probe fails

[Roy Spliet]

Hello Aaron,

Thanks for your insights. A follow-up query and some observations in-line.

Op 12-04-2021 om 20:06 schreef Aaron Plattner:
> On 4/10/21 1:48 PM, Roy Spliet wrote:
> > Op 10-04-2021 om 20:23 schreef Lukas Wunner:
> > > On Sat, Apr 10, 2021 at 04:51:27PM +0100, Roy Spliet wrote:
> > > > Can I ask someone with more
> > > > technical knowledge of snd_hda_intel and vgaswitcheroo to brainstorm 
> > > > about
> > > > the possible challenges of nouveau taking matters into its own hand 
> > > > rather
> > > > than keeping this PCI quirk around?
> > >
> > > It sounds to me like the HDA is not powered if no cable is plugged in.
> > > What is reponsible then for powering it up or down, firmware code on
> > > the GPU or in the host's BIOS?
> >
> > Sometimes the BIOS, but definitely unconditionally the PCI quirk code: 
> > https://github.com/torvalds/linux/blob/master/drivers/pci/quirks.c#L5289
> >
> > (CC Aaron Plattner)
>
> My basic understanding is that the audio function stops responding 
> whenever the graphics function is powered off. So the requirement here 
> is that the audio driver can't try to talk to the audio function while 
> the graphics function is asleep, and must trigger a graphics function 
> wakeup before trying to communicate with the audio function.

I believe that vgaswitcheroo takes care of this for us.

> I think 
> there are also requirements about the audio function needing to be awake 
> when the graphics driver is updating the ELD, but I'm not sure.
>
> This is harder on Windows because the audio driver lives in its own 
> little world doing its own thing but on Linux we can do better.
>
> > > Ideally, we should try to find out how to control HDA power from the
> > > operating system rather than trying to cooperate with whatever firmware
> > > is doing.  If we have that capability, the OS should power the HDA up
> > > and down as it sees fit.
>
> After system boot, I don't think there's any firmware involved, but I'm 
> not super familiar with the low-level details and it's possible the 
> situation changed since I last looked at it.
>
> I think the problem with having nouveau write this quirk is that the 
> kernel will need to re-probe the PCI device to notice that it has 
> suddenly become a multi-function device with an audio function, and 
> hotplug the audio driver. I originally looked into trying to do that but 
> it was tricky because the PCI subsystem didn't really have a mechanism 
> for a single-function device to become a multi-function device on the 
> fly and it seemed easier to enable it early on during bus enumeration. 
> That way the kernel sees both functions all the time without anything 
> else having to be special about this configuration.

Right, so for a little more context: a while ago I noticed that my 
laptop (lucky me, Asus K501UB) has a 940M with HDA but no codec. Seems 
legit, given how this GPU has no displays attached; they're all hooked 
up to the Intel integrated GPU. That threw off the snd_hda_intel 
mid-probe, and as a result didn't permit runpm, keeping the entire GPU, 
PCIe bus and thus the CPU package awake. A bit of hackerly later we 
decided to continue probing without a codec, and now my laptop is happy, 
but...
A new problem popped up with several other NVIDIA GPUs that expose their 
HDA subdevice, but somehow its inaccessible. Relevant lines from a 
users' log:

[    3.031222] MXM: GUID detected in BIOS
[    3.031280] ACPI BIOS Error (bug): AE_AML_PACKAGE_LIMIT, Index 
(0x000000003) is beyond end of object (length 0x0) (20200925/exoparg2-393)
[    3.031352] ACPI Error: Aborting method \_SB.PCI0.GFX0._DSM due to 
previous error (AE_AML_PACKAGE_LIMIT) (20200925/psparse-529)
[    3.031419] ACPI: \_SB_.PCI0.GFX0: failed to evaluate _DSM (0x300b)
[    3.031424] ACPI Warning: \_SB.PCI0.GFX0._DSM: Argument #4 type 
mismatch - Found [Buffer], ACPI requires [Package] (20200925/nsarguments-61)
[    3.031619] pci 0000:00:02.0: optimus capabilities: enabled, status 
dynamic power,
[    3.031667] ACPI BIOS Error (bug): AE_AML_PACKAGE_LIMIT, Index 
(0x000000003) is beyond end of object (length 0x0) (20200925/exoparg2-393)
[    3.031731] ACPI Error: Aborting method \_SB.PCI0.GFX0._DSM due to 
previous error (AE_AML_PACKAGE_LIMIT) (20200925/psparse-529)
[    3.031791] ACPI Error: Aborting method \_SB.PCI0.PEG0.PEGP._DSM due 
to previous error (AE_AML_PACKAGE_LIMIT) (20200925/psparse-529)
[    3.031856] ACPI: \_SB_.PCI0.PEG0.PEGP: failed to evaluate _DSM (0x300b)
[    3.031859] ACPI Warning: \_SB.PCI0.PEG0.PEGP._DSM: Argument #4 type 
mismatch - Found [Buffer], ACPI requires [Package] (20200925/nsarguments-61)
[    3.032058] pci 0000:01:00.0: optimus capabilities: enabled, status 
dynamic power,
[    3.032061] VGA switcheroo: detected Optimus DSM method 
\_SB_.PCI0.PEG0.PEGP handle
[    3.032323] checking generic (d0000000 410000) vs hw (f6000000 1000000)
[    3.032325] checking generic (d0000000 410000) vs hw (e0000000 10000000)
[    3.032326] checking generic (d0000000 410000) vs hw (f0000000 2000000)
[    3.032410] nouveau 0000:01:00.0: NVIDIA GK107 (0e71f0a2)
[    3.042385] nouveau 0000:01:00.0: bios: version 80.07.a0.00.11
--- snip ---
[    8.951478] snd_hda_intel 0000:01:00.1: can't change power state from 
D3cold to D0 (config space inaccessible)
[    8.951509] snd_hda_intel 0000:01:00.1: can't change power state from 
D3hot to D0 (config space inaccessible)
[    8.951608] snd_hda_intel 0000:01:00.1: Disabling MSI
[    8.951621] snd_hda_intel 0000:01:00.1: Handle vga_switcheroo audio 
client
[    8.952461] snd_hda_intel 0000:00:1b.0: bound 0000:00:02.0 (ops 
i915_audio_component_bind_ops [i915])
[    8.952642] snd_hda_intel 0000:01:00.1: number of I/O streams is 30, 
forcing separate stream tags

Now I don't know what's going on, but the snd_hda_intel messages are 
ominous. And so are the ACPI warnings. But I don't know how much these 
two are related.

You say that it is desirable to switch on HDA at boot-time because the 
PCI subsystem doesn't play nicely with changing a device to 
multi-function. That rules out the option of only enabling the HDA 
device once a cable is plugged in. Are there any other trap doors that 
snd_hda_intel needs to navigate around to make this work fault free on 
all hardware, such as:
- Codecs not revealing themselves until a display is plugged in, 
requiring perhaps a "codec reprobe" and "codec remove" event from 
nouveau/rm to snd_hda_intel,
- Borked BIOSes just blindly assigning the MMIO space of the HDA device 
to another device, or nothing at all,
- ... other things that might give any of us nightmares and heart burn?

Thanks!

Roy

> -- Aaron
>
> > > Thanks,
> > >
> > > Lukas