[Nouveau] [RFC PATCH] pci: prevent putting pcie devices into lower device states on certain intel bridges

[Karol Herbst]

On Mon, Sep 30, 2019 at 10:05 AM Mika Westerberg
<mika.westerberg at linux.intel.com> wrote:
>
> Hi Karol,
>
> On Fri, Sep 27, 2019 at 11:53:48PM +0200, Karol Herbst wrote:
> > > What exactly is the serious issue?  I guess it's that the rescan
> > > doesn't detect the GPU, which means it's not responding to config
> > > accesses?  Is there any timing component here, e.g., maybe we're
> > > missing some delay like the ones Mika is adding to the reset paths?
> >
> > When I was checking up on some of the PCI registers of the bridge
> > controller, the slot detection told me that there is no device
> > recognized anymore. I don't know which register it was anymore, though
> > I guess one could read it up in the SoC spec document by Intel.
> >
> > My guess is, that the bridge controller fails to detect the GPU being
> > here or actively threw it of the bus or something. But a normal system
> > suspend/resume cycle brings the GPU back online (doing a rescan via
> > sysfs gets the device detected again)
>
> Can you elaborate a bit what kind of scenario the issue happens (e.g
> steps how it reproduces)? It was not 100% clear from the changelog. Also
> what the result when the failure happens?
>

yeah, I already have an updated patch in the works which also does the
rework Bjorn suggested. Had no time yet to test if I didn't mess it
up.

I am also thinking of adding a kernel parameter to enable this
workaround on demand, but not quite sure on that one yet.

> I see there is a script that does something but unfortunately I'm not
> fluent in Python so can't extract the steps how the issue can be
> reproduced ;-)
>
> One thing that I'm working on is that Linux PCI subsystem misses certain
> delays that are needed after D3cold -> D0 transition, otherwise the
> device and/or link may not be ready before we access it. What you are
> experiencing sounds similar. I wonder if you could try the following
> patch and see if it makes any difference?
>
> https://patchwork.kernel.org/patch/11106611/

I think I already tried this path. The problem isn't that the device
isn't accessible too late, but that it seems that the device
completely falls off the bus. But I can retest again just to be sure.