Re: [PATCH 0/2] PCI/AER: Consistently use _OSC to determine who owns AER

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Nov 15, 2018 at 05:16:01PM -0600, Alexandru Gagniuc wrote:
> Thanks to Keith for pointing out that it doesn't make sense to disable
> AER services when only one device has a FIRMWARE_FIRST HEST.
> 
> AER ownership is an interesting issue brought in by FFS (firmware-first)
> model. In a nutshell if FFS handles AER, then OS should not touch any
> of the AER bits. FW might set things up so that it receives AER
> notifications via SMI. It's theoretically possible to receive SCIs,
> but the exact mechanism is platform-dependent. OS touching AER bits
> when firmware owns them may interfere with these notifications.
> 
> The ACPI mechanism for negotiating control of AER is _OSC, and is
> described in detail in ACPI 6.2 Ch. 6.2.11.3. _OSC is negotiated at
> the root bus level. Any root port, switch, or endpoint under the bus
> would have its AER ownership negotiated in one _OSC call.
> 
> Then there is HEST, which is part of ACPI Platform Error Interfaces
> (APEI). HEST tables describe the errors that FW may report to the OS.
> A types 6,7 and 7 HEST tables describe AER errors from PCIe devices.
> As part of this description, we're told if the error source is FFS.
> 
> Information in HEST seems to be redundant, as each error reported by
> FW will have a CPER table that describes it in detail.
> 
> Because HEST describes an error source as firmware-first or not, we've
> taken this to mean ownership of AER. Because AER ownership and error
> reporting are coupled, _OSC and HEST usually agree on the matter of
> ownership. However, that doesn't seem to be required by ACPI.
> 
> I've asked around a few people at Dell and they unanimously agree that
> _OSC is the correct way to determine ownership of AER. In linux, we
> use the result of _OSC to enable AER services, but we use HEST to
> determine AER ownership. That's inconsistent. This series drops the
> use of HEST in favor of _OSC.
> 
> [1] https://lkml.org/lkml/2018/11/15/62
> 
> Alexandru Gagniuc (2):
>   PCI/AER: Do not use APEI/HEST to disable AER services globally
>   PCI/AER: Determine AER ownership based on _OSC instead of HEST
> 
>  drivers/acpi/pci_root.c  |  9 +----
>  drivers/pci/pcie/aer.c   | 82 ++--------------------------------------
>  include/linux/pci-acpi.h |  6 ---
>  3 files changed, 5 insertions(+), 92 deletions(-)

I'm pretty sure we do need to do something here, but there was quite a
lot of discussion that didn't seem to really get resolved, so I'm
dropping these for now.

Please repost them with any relevant updates and we'll see if we can
get a consensus that we're going the right direction.

Bjorn