[Nouveau] [PATCH] PCI: Expose hidden NVIDIA HDA controllers

[Daniel Drake]

On Fri, Jun 14, 2019 at 4:03 AM Bjorn Helgaas <helgaas at kernel.org> wrote:
> Is it possible that this works on Windows but not Linux because they
> handle ACPI hotplug slightly differently?
>
> Martin did some nice debug [1] and found that _DSM, _PS0, and _PS3
> functions write the config bit at 0x488.  The dmesg log [2] from
> zigarrre seems to show that _OSC failed (which I *think* means we
> won't use pciehp) and that there's a slot registered by acpiphp.
>
> Maybe this works on Windows via an ACPI hotplug event that runs AML to
> flip the 0x488 bit and rescans the bus?  That would make more sense to
> me than the idea that the Nvidia driver does it.  I could imagine the
> driver flipping the bit, but the bus rescan seems like it would be out
> of the driver's purview.

Oh, I had missed that part of the conversation. I checked on the Acer
Predator G3-572 and I was able to find ACPI code to manipulate the
magic bit, however I can't see any linkage to _DSM methods, and the
code looks like it would only be run on power-up.

I modified the DSDT to avoid the codepaths that tweak the bit, booted
Windows and confirmed that change had taken effect. Then I installed
the nvidia driver and observed that the magic bit was being
manipulated according to HDMI cable status.

So I believe the nvidia Windows driver does not rely on ACPI for this
to work. It presumably does it directly and causes rescans, as evil as
that sounds. I added more details to the bug report.

> The dmesg log also shows some _DSM warnings; are these correlated with
> cable plug/unplug?  Is there some acpiphp debug we can turn on to get
> more visibility into hotplug events and how we handle them?

I scattered a load of debug messages around the ACPI & PCIe hotplug
code but nothing gets hit. I don't think this is architected to be
handled by existing PCI hotplug mechanisms.

I don't have any _DSM warnings on this product, even after connecting
and disconnecting the HDMI cable.

Thanks
Daniel