Re: [PATCH v3 0/2] PCI/ERR: Allow Native AER/DPC using _OSC

Derrick, Jonathan
2020-05-22T17:23:34Z

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@lixom.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@lixom.net>
> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
>
>
> Thanks,
> Jon

Hi Bjorn,

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.

Jon