linux-kernel - Re: [PATCH v17 09/12] PCI/AER: Allow clearing Error Status Register in FF mode

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Message-ID: <20200312223222.GA200236@google.com>
Date:   Thu, 12 Mar 2020 17:32:22 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Austin.Bolen@...l.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, ashok.raj@...el.com
Subject: Re: [PATCH v17 09/12] PCI/AER: Allow clearing Error Status Register
 in FF mode

On Thu, Mar 12, 2020 at 02:59:15PM -0700, Kuppuswamy Sathyanarayanan wrote:
> Hi Bjorn,
> 
> On 3/12/20 12:53 PM, Bjorn Helgaas wrote:
> > On Wed, Mar 11, 2020 at 04:07:59PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > > On 3/11/20 3:23 PM, Bjorn Helgaas wrote:
> > > > Is any synchronization needed here between the EDR path and the
> > > > hotplug/enumeration path?
> > > If we want to follow the implementation note step by step (in
> > > sequence) then we need some synchronization between EDR path and
> > > enumeration path. But if it's OK to achieve the same end result by
> > > following steps out of sequence then we don't need to create any
> > > dependency between EDR and enumeration paths. Currently we follow
> > > the latter approach.
> > What would the synchronization look like?
> we might need some way to disable the enumeration path till
> we get response from firmware.
> 
> In native hot plug case, I think we can do it in two ways.
> 
> 1. Disable hotplug notification in slot ctl registers.
>     (pcie_disable_notification())
> 2. Some how block hotplug driver from processing the new
>     events (not sure how feasible its).
> 
> Following method 1 would be easy, But I am not sure whether
> its alright to disable them randomly. I think, unless we
> clear the status as well, we might get some issues due to stale
> notification history.
> 
> For ACPI event case, I am not sure whether we have some
> communication protocol in place to disable receiving ACPI
> events temporarily.
> 
> For polling model, we need to disable to the polling
> timer thread till we receive _OST response from firmware.
> > 
> > Ideally I think it would be better to follow the order in the
> > flowchart if it's not too onerous.
> None of the above changes will be pretty and I think it will
> not be simple as well.
> >   That will make the code easier to
> > understand.  The current situation with this dependency on pciehp and
> > what it will do leaves a lot of things implicit.
> > 
> > What happens if CONFIG_PCIE_EDR=y but CONFIG_HOTPLUG_PCI_PCIE=n?
> > 
> > IIUC, when DPC triggers, pciehp is what fields the DLLSC interrupt and
> > unbinds the drivers and removes the devices.
> 
> >  If that doesn't happen, and Linux clears the DPC trigger to bring
> >  the link back up, will those drivers try to operate uninitialized
> >  devices?
>
> I don't think this will happen. In DPC reset_link before we bring up
> the device we wait for link to go down first using
> pcie_wait_for_link(pdev, false) function.

I understand that, but these child devices were reset when DPC
disabled the link.  When the link comes back up, their BARs contain
zeros.

If CONFIG_HOTPLUG_PCI_PCIE=y, the DLLSC interrupt will cause pciehp to
unbind the driver.  It seems like the unbind races with the EDR notify
handler.  If pciehp unbinds the driver before edr_handle_event() calls
pcie_do_recovery(), this seems fine -- we'll call dpc_reset_link(),
which brings up the link, we won't call any driver callbacks because
there's no driver, and another DLLSC interrupt will cause pciehp to
re-enumerate, which will re-initialize the device, then rebind the
driver.

If the EDR notify handler runs before pciehp unbinds the driver,
couldn't EDR bring up the link and call driver .mmio_enabled() before
the device has been initialized?

If CONFIG_HOTPLUG_PCI_PCIE=n and CONFIG_HOTPLUG_PCI_ACPI=y, I could
believe that the situations are similar to the above.

What if CONFIG_HOTPLUG_PCI_PCIE=n and CONFIG_HOTPLUG_PCI_ACPI=n?  Then
I assume there's nothing to unbind the driver, so pcie_do_recovery()
will call the driver .mmio_enabled() and other recovery callbacks on a
device that hasn't been initialized?