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

[Bjorn Helgaas <helgaas@kernel.org>]

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