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
