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

From: Olof Johansson <olof@lixom.net>
To: sathyanarayanan.kuppuswamy@linux.intel.com
Cc: Bjorn Helgaas <helgaas@kernel.org>,
	Keith Busch <keith.busch@intel.com>,
	linux-pci@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] PCI/DPC: Add pcie_ports=dpc-native parameter to bring back old behavior
Date: Thu, 7 Nov 2019 15:59:43 -0800	[thread overview]
Message-ID: <CAOesGMi2hnEhZvvcWg44LzjAr44LhzQ58s=u=THpfn=RRLLA7w@mail.gmail.com> (raw)
In-Reply-To: <43b431b6-f544-f9f0-d6f3-f383d7b882b9@linux.intel.com>

On Thu, Nov 7, 2019 at 2:07 PM Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>
>
> On 11/7/19 12:09 PM, Olof Johansson wrote:
> > On Thu, Nov 7, 2019 at 12:02 PM Kuppuswamy Sathyanarayanan
> > <sathyanarayanan.kuppuswamy@linux.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.
>
> Got it. I don't know whether its good to add hacks to support products
> that does not follow spec.
> But, do you think it would be useful to add some kind of warning message
> when this option is
> enabled ? May be it could be useful in debugging.

It's not a "hack", it is fixing a regression in behavior because of
changed assumptions by the kernel.

We're pretty clear as a kernel community: We don't regress our users.
So, in cases like these, we need to make sure we allow people to use
their hardware the same way they used it with an older kernel.

A printk() to indicate that this mode is enabled could be useful, if
nothing else to make sure that the pre-eed85ff4c0da7 behavior is
enabled without having to grep /proc/cmdline.

-Olof