Re: [Bug 215525] New: HotPlug does not work on upstream kernel 5.17.0-rc1

From: Bjorn Helgaas <helgaas@kernel.org>
To: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Cc: linux-pci@vger.kernel.org,
	Blazej Kucman <blazej.kucman@intel.com>,
	Hans de Goede <hdegoede@redhat.com>,
	Lukas Wunner <lukas@wunner.de>,
	Naveen Naidu <naveennaidu479@gmail.com>,
	Keith Busch <kbusch@kernel.org>,
	Nirmal Patel <nirmal.patel@linux.intel.com>,
	Jonathan Derrick <jonathan.derrick@linux.dev>
Subject: Re: [Bug 215525] New: HotPlug does not work on upstream kernel 5.17.0-rc1
Date: Thu, 27 Jan 2022 20:52:12 -0600	[thread overview]
Message-ID: <20220128025212.GA152555@bhelgaas> (raw)
In-Reply-To: <20220127154615.00003df8@linux.intel.com>

On Thu, Jan 27, 2022 at 03:46:15PM +0100, Mariusz Tkaczyk wrote:
> ...
> Thanks for your suggestions. Blazej did some tests and results were
> inconclusive. He tested it on two same platforms. On the first one it
> didn't work, even if he reverted all suggested patches. On the second
> one hotplugs always worked.
> 
> He noticed that on first platform where issue has been found initally,
> there was boot parameter "pci=nommconf". After adding this parameter
> on the second platform, hotplugs stopped working too.
> 
> Tested on tag pci-v5.17-changes. He have CONFIG_HOTPLUG_PCI_PCIE
> and CONFIG_DYNAMIC_DEBUG enabled in config. He also attached two dmesg
> logs to bugzilla with boot parameter 'dyndbg="file pciehp* +p" as
> requested. One with "pci=nommconf" and one without.
> 
> Issue seems to related to "pci=nommconf" and it is probably caused
> by change outside pciehp.

Maybe I'm missing something.  If I understand correctly, the problem
has nothing to do with the kernel version (correct me if I'm wrong!)

PCIe native hotplug doesn't work when booted with "pci=nommconf".
When using "pci=nommconf", obviously we can't access the extended PCI
config space (offset 0x100-0xfff), so none of the extended
capabilities are available.

In that case, we don't even ask the platform for control of PCIe
hotplug via _OSC.  From the dmesg diff from normal (working) to
"pci=nommconf" (not working):

  -Command line: BOOT_IMAGE=/boot/vmlinuz-smp ...
  +Command line: BOOT_IMAGE=/boot/vmlinuz-smp pci=nommconf ...
  ...
  -acpi PNP0A08:00: _OSC: OS supports [ExtendedConfig ASPM ClockPM Segments MSI HPX-Type3]
  -acpi PNP0A08:00: _OSC: platform does not support [AER LTR]
  -acpi PNP0A08:00: _OSC: OS now controls [PCIeHotplug PME PCIeCapability]
  +acpi PNP0A08:00: _OSC: OS supports [ASPM ClockPM Segments MSI HPX-Type3]
  +acpi PNP0A08:00: _OSC: not requesting OS control; OS requires [ExtendedConfig ASPM ClockPM MSI]
  +acpi PNP0A08:00: MMCONFIG is disabled, can't access extended PCI configuration space under this bridge.

Why are you using "pci=nommconf"?  As far as I know, there's no reason
to use that except to work around some kind of defect.

Bjorn