linux-kernel - Re: [PATCH v4] pci: prevent putting nvidia GPUs into lower device states on certain intel bridges

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Message-ID: <CACO55ttBkZD9dm0Y_jT931NnzHHtDFyLz28aoo+ZG0pnLzPgbA@mail.gmail.com>
Date:   Thu, 21 Nov 2019 17:06:09 +0100
From:   Karol Herbst <kherbst@...hat.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Mika Westerberg <mika.westerberg@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lyude Paul <lyude@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        nouveau <nouveau@...ts.freedesktop.org>,
        Dave Airlie <airlied@...il.com>,
        Mario Limonciello <Mario.Limonciello@...l.com>
Subject: Re: [PATCH v4] pci: prevent putting nvidia GPUs into lower device
 states on certain intel bridges

On Thu, Nov 21, 2019 at 4:47 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Thu, Nov 21, 2019 at 1:53 PM Karol Herbst <kherbst@...hat.com> wrote:
> >
> > On Thu, Nov 21, 2019 at 12:46 PM Mika Westerberg
> > <mika.westerberg@...el.com> wrote:
> > >
> > > On Thu, Nov 21, 2019 at 12:34:22PM +0100, Rafael J. Wysocki wrote:
> > > > On Thu, Nov 21, 2019 at 12:28 PM Mika Westerberg
> > > > <mika.westerberg@...el.com> wrote:
> > > > >
> > > > > On Wed, Nov 20, 2019 at 11:29:33PM +0100, Rafael J. Wysocki wrote:
> > > > > > > last week or so I found systems where the GPU was under the "PCI
> > > > > > > Express Root Port" (name from lspci) and on those systems all of that
> > > > > > > seems to work. So I am wondering if it's indeed just the 0x1901 one,
> > > > > > > which also explains Mikas case that Thunderbolt stuff works as devices
> > > > > > > never get populated under this particular bridge controller, but under
> > > > > > > those "Root Port"s
> > > > > >
> > > > > > It always is a PCIe port, but its location within the SoC may matter.
> > > > >
> > > > > Exactly. Intel hardware has PCIe ports on CPU side (these are called
> > > > > PEG, PCI Express Graphics, ports), and the PCH side. I think the IP is
> > > > > still the same.
> > > > >
> >
> > yeah, I meant the bridge controller with the ID 0x1901 is on the CPU
> > side. And if the Nvidia GPU is on a port on the PCH side it all seems
> > to work just fine.
>
> But that may involve different AML too, may it not?
>
> > > > > > Also some custom AML-based power management is involved and that may
> > > > > > be making specific assumptions on the configuration of the SoC and the
> > > > > > GPU at the time of its invocation which unfortunately are not known to
> > > > > > us.
> > > > > >
> > > > > > However, it looks like the AML invoked to power down the GPU from
> > > > > > acpi_pci_set_power_state() gets confused if it is not in PCI D0 at
> > > > > > that point, so it looks like that AML tries to access device memory on
> > > > > > the GPU (beyond the PCI config space) or similar which is not
> > > > > > accessible in PCI power states below D0.
> > > > >
> > > > > Or the PCI config space of the GPU when the parent root port is in D3hot
> > > > > (as it is the case here). Also then the GPU config space is not
> > > > > accessible.
> > > >
> > > > Why would the parent port be in D3hot at that point?  Wouldn't that be
> > > > a suspend ordering violation?
> > >
> > > No. We put the GPU into D3hot first, then the root port and then turn
> > > off the power resource (which is attached to the root port) resulting
> > > the topology entering D3cold.
> > >
> >
> > If the kernel does a D0 -> D3hot -> D0 cycle this works as well, but
> > the power savings are way lower, so I kind of prefer skipping D3hot
> > instead of D3cold. Skipping D3hot doesn't seem to make any difference
> > in power savings in my testing.
>
> OK
>
> What exactly did you do to skip D3cold in your testing?
>

For that I poked into the PCI registers directly and skipped doing the
ACPI calls and simply checked for the idle power consumption on my
laptop. But I guess I should retest with calling pci_d3cold_disable
from nouveau instead? Or is there a different preferable way of
testing this?