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Subject: Re: pciehp 0000:00:1c.0:pcie004: Timeout on hotplug command 0x1038
 (issued 65284 msec ago)
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     okaya@codeaurora.org, Bjorn Helgaas <bhelgaas@google.com>,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        Lukas Wunner <lukas@wunner.de>
References: <8770820b-85a0-172b-7230-3a44524e6c9f@molgen.mpg.de>
 <20180427192207.GG8199@bhelgaas-glaptop.roam.corp.google.com>
 <43b8ab4a-f8ee-dc96-40ec-e6fdfedd8309@molgen.mpg.de>
 <20180504024527.GE15790@bhelgaas-glaptop.roam.corp.google.com>
 <aa72d3ba089c76449600c22b58a2cafe@codeaurora.org>
 <20180504133327.GF15790@bhelgaas-glaptop.roam.corp.google.com>
 <20180507213344.GA133147@bhelgaas-glaptop.roam.corp.google.com>
 <903e7c20-fdd7-9cbf-debb-a90e70240c7c@molgen.mpg.de>
 <20180508123432.GJ161390@bhelgaas-glaptop.roam.corp.google.com>
From:   Paul Menzel <pmenzel+linux-pci@molgen.mpg.de>
Message-ID: <302b3ac9-b34f-c061-8c59-6b3b0c56b1f7@molgen.mpg.de>
Date:   Tue, 8 May 2018 15:22:54 +0200
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Dear Bjorn,


Am 08.05.2018 um 14:34 schrieb Bjorn Helgaas:
> On Tue, May 08, 2018 at 08:59:34AM +0200, Paul Menzel wrote:

>> 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?

No, not more then that. But “oh, that’s not good” fits in my opinion. My 
impression was, that’s how most stable patches get in.


Kind regards,

Paul