Re: [PATCH v2 1/3] PCI/DPC: Request DPC only if also requesting AER

[Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 2/26/24 8:33 AM, Bjorn Helgaas wrote:
> On Mon, Feb 26, 2024 at 07:46:05AM -0800, Kuppuswamy Sathyanarayanan wrote:
>> On 2/26/24 7:18 AM, Bjorn Helgaas wrote:
>>> On Sun, Feb 25, 2024 at 11:46:07AM -0800, Kuppuswamy Sathyanarayanan wrote:
>>>> On 2/22/24 2:15 PM, Bjorn Helgaas wrote:
>>>>> From: Bjorn Helgaas <bhelgaas@google.com>
>>>>>
>>>>> When booting with "pci=noaer", we don't request control of AER, but we
>>>>> previously *did* request control of DPC, as in the dmesg log attached at
>>>>> the bugzilla below:
>>>>>
>>>>>   Command line: ... pci=noaer
>>>>>   acpi PNP0A08:00: _OSC: OS supports [ExtendedConfig ASPM ClockPM Segments MSI EDR HPX-Type3]
>>>>>   acpi PNP0A08:00: _OSC: OS now controls [PCIeHotplug SHPCHotplug PME PCIeCapability LTR DPC]
>>>>>
>>>>> That's illegal per PCI Firmware Spec, r3.3, sec 4.5.1, table 4-5, which
>>>>> says:
>>>>>
>>>>>   If the operating system sets this bit [OSC_PCI_EXPRESS_DPC_CONTROL], it
>>>>>   must also set bit 7 of the Support field (indicating support for Error
>>>>>   Disconnect Recover notifications) and bits 3 and 4 of the Control field
>>>>>   (requesting control of PCI Express Advanced Error Reporting and the PCI
>>>>>   Express Capability Structure).
>>>> IIUC, this dependency is discussed in sec 4.5.2.4. "Dependencies
>>>> Between _OSC Control Bits".
>>>>
>>>> Because handling of Downstream Port Containment has a dependency on
>>>> Advanced Error Reporting, the operating system is required to
>>>> request control over Advanced Error Reporting (bit 3 of the Control
>>>> field) while requesting control over Downstream Port Containment
>>>> Configuration (bit 7 of the Control field). If the operating system
>>>> attempts to claim control of Downstream Port Containment
>>>> Configuration without also claiming control over Advanced Error
>>>> Reporting, firmware is required to refuse control of the feature
>>>> being illegally claimed and mask the corresponding bit.  Firmware is
>>>> required to maintain ownership of Advanced Error Reporting if it
>>>> retains ownership of Downstream Port Containment Configuration.  If
>>>> the operating system sets bit 7 of the Control field, it must set
>>>> bit 7 of the Support field, indicating support for the Error
>>>> Disconnect Recover event.
>>> So I guess you're suggesting that there are two defects here?
>>>
>>>   1) Linux requested DPC control without requesting AER control.
>>>
>>>   2) Platform granted DPC control when it shouldn't have.
>>>
>>> I do agree with that, but obviously we can only fix 1) in Linux.
>> Sorry, maybe my comment was not clear. I was just suggesting to
>> change the spec reference from r3.3, sec 4.5.1, table 4-5 to r3.3,
>> sec 4.5.2.4 "Dependencies Between _OSC Control Bits".
> The requirement that the OS request AER control whenever it requests
> DPC control is mentioned in both sec 4.5.1 and sec 4.5.2.4.  IMO sec
> 4.5.2.4 should not exist because the per-bit table in sec 4.5.1 is a
> better place for implementation guidance.  4.5.2.4 is easy to miss,
> mostly redundant, and hard to integrate with the 4.5.1 table.
>
> What advantage do you see for citing 4.5.2.4 instead of 4.5.1?  The
> only real difference I see is that it also points out a firmware
> problem.  I don't think the extra text is worth it since it doesn't
> motivate the Linux change.


My thinking is, since the fix is related to the dependency between
_OSC control bits (AER & DPC), and there is a special section in the
spec which discuss it, I thought it is better to  quote it.

But I get your point. I think either if fine.

>
> Bjorn

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer