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

Kuppuswamy, Sathyanarayanan
2020-05-22T20:48:14Z

Hi Bjorn, Derrick,

On 5/22/20 12:46 PM, Bjorn Helgaas wrote:
> 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@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>
>>
>> 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.

Sorry for the delay.

I was just waiting to see whether we get any issues with merging
following commit.

commit c100beb9ccfb98e2474586a4006483cbf770c823
Author: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Date: Mon Apr 27 18:25:13 2020 -0500

PCI/AER: Use only _OSC to determine AER ownership

Since I did not see any email reporting any issues about it,
I will work on follow up patch.

>
> Bjorn
>