linux-kernel - Re: [PATCH v1 0/2] PCI: hotplug: Add checks to avoid doing hotplug on PCIe Upstream Ports

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Date:   Tue, 22 Nov 2022 09:04:55 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH v1 0/2] PCI: hotplug: Add checks to avoid doing hotplug
 on PCIe Upstream Ports

On Mon, Nov 21, 2022 at 07:13:15PM +0100, Rafael J. Wysocki wrote:
> PCIe Upstream Ports are not hotplug-capable by definition, but it turns out
> that in some cases, if the system is configured in a particularly interesting
> way, the kernel may be made attempt to operate an Upstream Port as a hotplug
> one which causes functional issues to appear.
> 
> The following 2 patches amend the code to prevent this behavior from occurring.

Both patches LGTM.

The spec reference for this change is PCIe r6.0.1 sec 7.5.3.2:

The Slot Implemented bit in the PCI Express Capabilities register
is only valid for Downstream Ports and undefined on Upstream Ports.

The Slot Capabilities / Control / Status registers are only operable
if the Slot Implemented bit is valid and set.  PCIe hotplug depends
on those registers.

(pcie_capability_reg_implemented() in drivers/pci/access.c returns false
for the Slot Capabilities / Control / Status registers unless the port
is a Root or Downstream Port.  Reads of those registers thus always
return 0.)

(Root Ports are Downstream Ports per the definition of "Downstream"
on page 94 of the PCIe r6.0.1 Base Spec.)

Thanks,

Lukas