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Date:   Tue, 8 May 2018 07:34:32 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Paul Menzel <pmenzel+linux-pci@molgen.mpg.de>
Cc:     okaya@codeaurora.org, Bjorn Helgaas <bhelgaas@google.com>,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        Lukas Wunner <lukas@wunner.de>
Subject: Re: pciehp 0000:00:1c.0:pcie004: Timeout on hotplug command 0x1038
 (issued 65284 msec ago)
Message-ID: <20180508123432.GJ161390@bhelgaas-glaptop.roam.corp.google.com>
References: <8770820b-85a0-172b-7230-3a44524e6c9f@molgen.mpg.de>
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On Tue, May 08, 2018 at 08:59:34AM +0200, Paul Menzel wrote:
> Dear Bjorn,
> 
> 
> Am 07.05.2018 um 23:33 schrieb Bjorn Helgaas:
> > On Fri, May 04, 2018 at 08:33:27AM -0500, Bjorn Helgaas wrote:
> > > commit b0d6f2230e12c85ae3b65a854a53c67c7c1f6406
> > > Author: Bjorn Helgaas <bhelgaas@google.com>
> > > Date:   Thu May 3 18:39:38 2018 -0500
> > > 
> > >      PCI: pciehp: Add quirk for Intel Command Completed erratum
> > >      The Intel CF118 erratum means the controller does not set the Command
> > >      Completed bit unless writes to the Slot Command register change "Control"
> > >      bits.  Command Completed is never set for writes that only change software
> > >      notification "Enable" bits.  This results in timeouts like this:
> > >        pciehp 0000:00:1c.0:pcie004: Timeout on hotplug command 0x1038 (issued 65284 msec ago)
> > >      When this erratum is present, avoid these timeouts by marking commands
> > >      "completed" immediately unless they change the "Control" bits.
> > >      Here's the text of the erratum from the Intel document:
> > >        CF118        PCIe Slot Status Register Command Completed bit not always
> > >                     updated on any configuration write to the Slot Control
> > >                     Register
> > >        Problem:     For PCIe root ports (devices 0 - 10) supporting hot-plug,
> > >                     the Slot Status Register (offset AAh) Command Completed
> > >                     (bit[4]) status is updated under the following condition:
> > >                     IOH will set Command Completed bit after delivering the new
> > >                     commands written in the Slot Controller register (offset
> > >                     A8h) to VPP. The IOH detects new commands written in Slot
> > >                     Control register by checking the change of value for Power
> > >                     Controller Control (bit[10]), Power Indicator Control
> > >                     (bits[9:8]), Attention Indicator Control (bits[7:6]), or
> > >                     Electromechanical Interlock Control (bit[11]) fields. Any
> > >                     other configuration writes to the Slot Control register
> > >                     without changing the values of these fields will not cause
> > >                     Command Completed bit to be set.
> > >                     The PCIe Base Specification Revision 2.0 or later describes
> > >                     the “Slot Control Register” in section 7.8.10, as follows
> > >                     (Reference section 7.8.10, Slot Control Register, Offset
> > >                     18h). In hot-plug capable Downstream Ports, a write to the
> > >                     Slot Control register must cause a hot-plug command to be
> > >                     generated (see Section 6.7.3.2 for details on hot-plug
> > >                     commands). A write to the Slot Control register in a
> > >                     Downstream Port that is not hotplug capable must not cause a
> > >                     hot-plug command to be executed.
> > >                     The PCIe Spec intended that every write to the Slot Control
> > >                     Register is a command and expected a command complete status
> > >                     to abstract the VPP implementation specific nuances from the
> > >                     OS software. IOH PCIe Slot Control Register implementation
> > >                     is not fully conforming to the PCIe Specification in this
> > >                     respect.
> > >        Implication: Software checking on the Command Completed status after
> > >                     writing to the Slot Control register may time out.
> > >        Workaround:  Software can read the Slot Control register and compare the
> > >                     existing and new values to determine if it should check the
> > >                     Command Completed status after writing to the Slot Control
> > >                     register.
> > >      Link: http://www.intel.com/content/www/us/en/processors/xeon/xeon-e7-v2-spec-update.html
> > >      Link: https://lkml.kernel.org/r/8770820b-85a0-172b-7230-3a44524e6c9f@molgen.mpg.de
> > >      Reported-by: Paul Menzel <pmenzel+linux-pci@molgen.mpg.de>
> > >      Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> > 
> > I applied this with Paul's tested-by on pci/hotplug for v4.18.
> 
> Thank you very much. Will this also be picked up by the stable Linux kernel
> series?

I did not tag it for stable because I didn't think it was a serious enough
problem, based on this from Documentation/process/stable-kernel-rules.rst:

 - It must fix a problem that causes a build error (but not for things
   marked CONFIG_BROKEN), an oops, a hang, data corruption, a real
   security issue, or some "oh, that's not good" issue.  In short, something
   critical.

I know I'm on the conservative end of the stable-tagging spectrum, so maybe
I could be convinced to add a stable tag.

My impression was that this bug caused annoying messages and annoying
delays of a couple seconds during shutdown and resume.  Is it more serious
than that?

Bjorn