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Date:   Fri, 1 May 2020 17:35:16 +0000
From:   "Derrick, Jonathan" <jonathan.derrick@...el.com>
To:     "helgaas@...nel.org" <helgaas@...nel.org>
CC:     "Patel, Mayurkumar" <mayurkumar.patel@...el.com>,
        "rajatja@...gle.com" <rajatja@...gle.com>,
        "fred@...dlawl.com" <fred@...dlawl.com>,
        "ruscur@...sell.cc" <ruscur@...sell.cc>,
        "oohall@...il.com" <oohall@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "sbobroff@...ux.ibm.com" <sbobroff@...ux.ibm.com>,
        "olof@...om.net" <olof@...om.net>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH v3 0/2] PCI/ERR: Allow Native AER/DPC using _OSC

On Fri, 2020-05-01 at 12:16 -0500, Bjorn Helgaas wrote:
> On Thu, Apr 30, 2020 at 12:46:07PM -0600, Jon Derrick wrote:
> > Hi Bjorn & Kuppuswamy,
> > 
> > I see a problem in the DPC ECN [1] to _OSC in that it doesn't give us a way to
> > determine if firmware supports _OSC DPC negotation, and therefore how to handle
> > DPC.
> > 
> > Here is the wording of the ECN that implies that Firmware without _OSC DPC
> > negotiation support should have the OSPM rely on _OSC AER negotiation when
> > determining DPC control:
> > 
> >   PCIe Base Specification suggests that Downstream Port Containment may be
> >   controlled either by the Firmware or the Operating System. It also suggests
> >   that the Firmware retain ownership of Downstream Port Containment if it also
> >   owns AER. When the Firmware owns Downstream Port Containment, it is expected
> >   to use the new "Error Disconnect Recover" notification to alert OSPM of a
> >   Downstream Port Containment event.
> > 
> > In legacy platforms, as bits in _OSC are reserved prior to implementation, ACPI
> > Root Bus enumeration will mark these Host Bridges as without Native DPC
> > support, even though the specification implies it's expected that AER _OSC
> > negotiation determines DPC control for these platforms. There seems to be a
> > need for a way to determine if the DPC control bit in _OSC is supported and
> > fallback on AER otherwise.
> > 
> > 
> > Currently portdrv assumes DPC control if the port has Native AER services:
> > 
> > static int get_port_device_capability(struct pci_dev *dev)
> > ...
> > 	if (pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DPC) &&
> > 	    pci_aer_available() &&
> > 	    (pcie_ports_dpc_native || (services & PCIE_PORT_SERVICE_AER)))
> > 		services |= PCIE_PORT_SERVICE_DPC;
> > 
> > Newer firmware may not grant OSPM DPC control, if for instance, it expects to
> > use Error Disconnect Recovery. However it looks like ACPI will use DPC services
> > via the EDR driver, without binding the full DPC port service driver.
> > 
> > 
> > If we change portdrv to probe based on host->native_dpc and not AER, then we
> > break instances with legacy firmware where OSPM will clear host->native_dpc
> > solely due to _OSC bits being reserved:
> > 
> > struct pci_bus *acpi_pci_root_create(struct acpi_pci_root *root,
> > ...
> > 	if (!(root->osc_control_set & OSC_PCI_EXPRESS_DPC_CONTROL))
> > 		host_bridge->native_dpc = 0;
> > 
> > 
> > 
> > So my assumption instead is that host->native_dpc can be 0 and expect Native
> > DPC services if AER is used. In other words, if and only if DPC probe is
> > invoked from portdrv, then it needs to rely on the AER dependency. Otherwise it
> > should be assumed that ACPI set up DPC via EDR. This covers legacy firmware.
> > 
> > However it seems like that could be trouble with newer firmware that might give
> > OSPM control of AER but not DPC, and would result in both Native DPC and EDR
> > being in effect.
> > 
> > 
> > Anyways here are two patches that give control of AER and DPC on the results of
> > _OSC. They don't mess with the HEST parser as I expect those to be removed at
> > some point. I need these for VMD support which doesn't even rely on _OSC, but I
> > suspect this won't be the last effort as we detangle Firmware First.
> > 
> > [1] https://members.pcisig.com/wg/PCI-SIG/document/12888
> 
> Hi Jon, I think we need to sort out the _OSC/FIRMWARE_FIRST patches
> from Alex and Sathy first, then see what needs to be done on top of
> those, so I'm going to push these off for a few days and they'll
> probably need a refresh.
> 
> Bjorn


Agreed, no need to merge now. Just wanted to bring up the DPC
ambiguity, which I think was first addressed by dpc-native:

commit 35a0b2378c199d4f26e458b2ca38ea56aaf2d9b8
Author: Olof Johansson <olof@...om.net>
Date:   Wed Oct 23 12:22:05 2019 -0700

    PCI/DPC: Add "pcie_ports=dpc-native" to allow DPC without AER control
    
    Prior to eed85ff4c0da7 ("PCI/DPC: Enable DPC only if AER is available"),
    Linux handled DPC events regardless of whether firmware had granted it
    ownership of AER or DPC, e.g., via _OSC.
    
    PCIe r5.0, sec 6.2.10, recommends that the OS link control of DPC to
    control of AER, so after eed85ff4c0da7, Linux handles DPC events only if it
    has control of AER.
    
    On platforms that do not grant OS control of AER via _OSC, Linux DPC
    handling worked before eed85ff4c0da7 but not after.
    
    To make Linux DPC handling work on those platforms the same way they did
    before, add a "pcie_ports=dpc-native" kernel parameter that makes Linux
    handle DPC events regardless of whether it has control of AER.
    
    [bhelgaas: commit log, move pcie_ports_dpc_native to drivers/pci/]
    Link: https://lore.kernel.org/r/20191023192205.97024-1-olof@lixom.net
    Signed-off-by: Olof Johansson <olof@...om.net>
    Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>


Thanks,
Jon

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