Re: [PATCH v3] pci: prevent putting nvidia GPUs into lower device states on certain intel bridges

[Karol Herbst]

On Tue, Oct 22, 2019 at 2:45 PM Mika Westerberg
<mika.westerberg@xxxxxxxxx> wrote:
>
> On Tue, Oct 22, 2019 at 11:16:14AM +0200, Karol Herbst wrote:
> > I think there is something I totally forgot about:
> >
> > When there was never a driver bound to the GPU, and if runtime power
> > management gets enabled on that device, runtime suspend/resume works
> > as expected (I am not 100% sure on if that always works, but I will
> > recheck that).
>
> AFAIK, if there is no driver bound to the PCI device it is left to D0
> regardless of the runtime PM state which could explain why it works in
> that case (it is never put into D3hot).
>
> I looked at the acpidump you sent and there is one thing that may
> explain the differences between Windows and Linux. Not sure if you were
> aware of this already, though. The power resource PGOF() method has
> this:
>
>    If (((OSYS <= 0x07D9) || ((OSYS == 0x07DF) && (_REV == 0x05)))) {
>       ...
>    }
>

I think this is the fallback to some older method of runtime
suspending the device, and I think it will end up touching different
registers on the bridge controller which do not show the broken
behaviour.

You'll find references to following variables which all cause a link
to be powered down: Q0L2 (newest), P0L2, P0LD (oldest, I think).

Maybe I remember incorrectly and have to read the code again... okay,
the fallback path uses P0LD indeed. That's actually the only register
of those being documented by Intel afaik.

> If I read it right, the later condition tries to detect Linux which
> fails nowadays but if you have acpi_rev_override in the command line (or
> the machine is listed in acpi_rev_dmi_table) this check passes and does
> some magic which is not clear to me. There is similar in PGON() side
> which is used to turn the device back on.
>
> You can check what actually happens when _ON()/_OFF() is called by
> passing something like below to the kernel command line:
>
>   acpi.trace_debug_layer=0x80 acpi.trace_debug_level=0x10 acpi.trace_method_name=\_SB.PCI0.PEG0.PG00._ON acpi.trace_state=method
>
> (See also Documentation/firmware-guide/acpi/method-tracing.rst).
>
> Trace goes to system dmesg.

This sounds to be very helpful, I'll give it a try.