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

[Rafael J. Wysocki]

On Wed, Nov 20, 2019 at 11:18 AM Mika Westerberg
<mika.westerberg@xxxxxxxxx> wrote:
>
> Hi Karol,
>
> On Tue, Nov 19, 2019 at 11:26:45PM +0100, Karol Herbst wrote:
> > On Tue, Nov 19, 2019 at 10:50 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > >
> > > [+cc Dave]
> > >
> > > On Thu, Oct 17, 2019 at 02:19:01PM +0200, Karol Herbst wrote:
> > > > Fixes state transitions of Nvidia Pascal GPUs from D3cold into higher device
> > > > states.
> > > >
> > > > v2: convert to pci_dev quirk
> > > >     put a proper technical explanation of the issue as a in-code comment
> > > > v3: disable it only for certain combinations of intel and nvidia hardware
> > > > v4: simplify quirk by setting flag on the GPU itself
> > >
> > > I have zero confidence that we understand the real problem, but we do
> > > need to do something with this.  I'll merge it for v5.5 if we get the
> > > minor procedural stuff below straightened out.
> > >
> >
> > Thanks, and I agree with your statement, but at this point I think
> > only Intel can help out digging deeper as I see no way to debug this
> > further.
>
> I don't have anything against this patch, as long as the quirk stays
> limited to the particular root port leading to the NVIDIA GPU. The
> reason why I think it should to be limited is that I'm pretty certain
> the problem is not in the root port itself. I have here a KBL based
> Thinkpad X1 Carbon 6th gen that can put the TBT controller into D3cold
> (it is connected to PCH root port) and it wakes up there just fine, so
> don't want to break that.
>
> Now, PCIe devices cannot go into D3cold all by themselves. They always
> need help from the platform side which is ACPI in this case. This is
> done by having the device to have _PR3 method that returns one or more
> power resources that the OS is supposed to turn off when the device is
> put into D3cold. All of that is implemented as form of ACPI methods that
> pretty much do the hardware specific things that are outside of PCIe
> spec to get the device into D3cold. At high level the _OFF() method
> causes the root port to broadcast PME_Turn_Off message that results the
> link to enter L2/3 ready, it then asserts PERST, configures WAKE (both
> can be GPIOs) and finally removes power (if the link goes into L3,
> otherwise it goes into L2).
>
> I think this is where the problem actually lies - the ASL methods that
> are used to put the device into D3cold and back. We know that in Windows
> this all works fine so unless Windows quirks the root port the same way
> there is another reason behind this.
>
> In case of Dell XPS 9560 (IIRC that's the machine you have) the
> corresponding power resource is called \_SB.PCI0.PEG0.PG00 and its
> _ON/_OFF methods end up calling PGON()/PGOF() accordingly. The methods
> itself do lots of things and it is hard to follow the dissassembled
> ASL which does not have any comments but there are couple of things that
> stand out where we may go into a different path. One of them is this in
> the PGOF() method:
>
>    If (((OSYS <= 0x07D9) || ((OSYS == 0x07DF) && (_REV == 0x05))))
>
> The ((OSYS == 0x07DF) && (_REV == 0x05)) checks specifically for Linux
> (see [1] and 18d78b64fddc ("ACPI / init: Make it possible to override
> _REV")) so it might be that Dell people tested this at some point in
> Linux as well. Added Mario in case he has any ideas.
>
> Previously I suggested you to try the ACPI method tracing to see what
> happens inside PGOF(). Did you have time to try it? It may provide more
> information about that is happening inside those methods and hopefully
> point us to the root cause.
>
> Also if you haven't tried already passing acpi_rev_override in the
> command line makes the _REV to return 5 so it should go into the "Linux"
> path in PGOF().

Oh, so does it look like we are trying to work around AML that tried
to work around some problematic behavior in Linux at one point?

> [1] https://www.kernel.org/doc/html/latest/firmware-guide/acpi/osi.html#do-not-use-rev