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Date:   Thu, 12 Apr 2018 08:39:54 -0600
From:   Keith Busch <keith.busch@intel.com>
To:     Sinan Kaya <okaya@codeaurora.org>
Cc:     Bjorn Helgaas <helgaas@kernel.org>,
        Oza Pawandeep <poza@codeaurora.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        Dongdong Liu <liudongdong3@huawei.com>,
        Wei Zhang <wzhang@fb.com>, Timur Tabi <timur@codeaurora.org>,
        Alex Williamson <alex.williamson@redhat.com>
Subject: Re: [PATCH v13 6/6] PCI/DPC: Do not do recovery for hotplug enabled
 system
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On Thu, Apr 12, 2018 at 10:34:37AM -0400, Sinan Kaya wrote:
> On 4/12/2018 10:06 AM, Bjorn Helgaas wrote:
> > 
> > I think the scenario you are describing is two systems that are
> > identical except that in the first, the endpoint is below a hotplug
> > bridge, while in the second, it's below a non-hotplug bridge.  There's
> > no physical hotplug (no drive removed or inserted), and DPC is
> > triggered in both systems.
> > 
> > I suggest that DPC should be handled identically in both systems:
> > 
> >   - The PCI core should have the same view of the endpoint: it should
> >     be removed and re-added in both cases (or in neither case).
> > 
> >   - The endpoint itself should not be able to tell the difference: it
> >     should see a link down event, followed by a link retrain, followed
> >     by the same sequence of config accesses, etc.
> > 
> >   - The endpoint driver should not be able to tell the difference,
> >     i.e., we should be calling the same pci_error_handlers callbacks
> >     in both cases.
> > 
> > It's true that in the non-hotplug system, pciehp probably won't start
> > re-enumeration, so we might need an alternate path to trigger that.
> > 
> > But that's not what we're doing in this patch.  In this patch we're
> > adding a much bigger difference: for hotplug bridges, we stop and
> > remove the hierarchy below the bridge; for non-hotplug bridges, we do
> > the AER-style flow of calling pci_error_handlers callbacks.
> 
> Our approach on V12 was to go to AER style recovery for all DPC events
> regardless of hotplug support or not. 
> 
> Keith was not comfortable with this approach. That's why, we special cased
> hotplug.
> 
> If we drop 6/6 on this patch on v13, we achieve this. We still have to
> take care of Keith's inputs on individual patches.
> 
> we have been struggling with the direction for a while.
> 
> Keith, what do you think?

My only concern was for existing production environments that use DPC
for handling surprise removal, and I don't wish to break the existing
uses.