Date:   Fri, 22 May 2020 14:46:16 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Derrick, Jonathan" <jonathan.derrick@...el.com>
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, May 22, 2020 at 05:23:31PM +0000, Derrick, Jonathan wrote:
> On Fri, 2020-05-01 at 11:35 -0600, Jonathan Derrick wrote:
> > 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>
> 
> Are you still thinking about removing the HEST parser?
> 
> For VMD we still need the ability to bind DPC if native_dpc==1.
> I think if we can do that, this set should still pretty much still
> apply with a modification to patch 2 to allow matching
> pcie_ports_dpc_native in dpc_probe.

Yes, I think we should remove the HEST firmware-first parsing, because
IIRC the spec really doesn't specify any action the OS should take
based on it.  I was thinking Sathy might update the patch, and it fell
off my radar.

Bjorn

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