linux-kernel - Re: [PATCH] PCI/DPC: Add pcie_ports=dpc-native parameter to bring back old behavior

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Message-ID: <CAOesGMhdAUKj9f0=sVwH7kffr=P-LqWWqKxKK3N3e0MpcjLExw@mail.gmail.com>
Date:   Thu, 7 Nov 2019 12:09:02 -0800
From:   Olof Johansson <olof@...om.net>
To:     sathyanarayanan.kuppuswamy@...ux.intel.com
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Keith Busch <keith.busch@...el.com>, linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI/DPC: Add pcie_ports=dpc-native parameter to bring
 back old behavior

On Thu, Nov 7, 2019 at 12:02 PM Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@...ux.intel.com> wrote:
>
> Hi,
>
> On 10/25/19 1:20 PM, Bjorn Helgaas wrote:
> > On Wed, Oct 23, 2019 at 12:22:05PM -0700, Olof Johansson wrote:
> >> In commit eed85ff4c0da7 ("PCI/DPC: Enable DPC only if AER is available"),
> >> the behavior was changed such that native (kernel) handling of DPC
> >> got tied to whether the kernel also handled AER. While this is what
> >> the standard recommends, there are BIOSes out there that lack the DPC
> >> handling since it was never required in the past.
> > Some systems do not grant OS control of AER via _OSC.  I guess the
> > problem is that on those systems, the OS DPC driver used to work, but
> > after eed85ff4c0da7, it does not.  Right?
>
> I need some clarification on this issue. Do you mean in these systems,
> firmware expects OS to handle DPC and AER, but it does not let OS know
> about it via _OSC ?

The OS and BIOS both assumed behavior as before eed85ff4c0da7 -- AER
handled by firmware but DPC handled by kernel.

I.e. a classic case of "Sure, the spec says this, but in reality
implementations relied on actual behavior", and someone had a
regression and need a way to restore previous behavior.


-Olof