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    SubjectRe: [RFC PATCH] pci: prevent putting pcie devices into lower device states on certain intel bridges
    On Mon, Sep 30, 2019 at 10:05 AM Mika Westerberg
    <mika.westerberg@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.

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