linux-kernel - Re: [RFC PATCH] pci: prevent putting pcie devices into lower device states on certain intel bridges

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Date:   Mon, 30 Sep 2019 11:05:34 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Karol Herbst <kherbst@...hat.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lyude Paul <lyude@...hat.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        nouveau <nouveau@...ts.freedesktop.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH] pci: prevent putting pcie devices into lower device
 states on certain intel bridges

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?

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/