[cc += Joel Mathew Thomas]

On Tue, Jun 10, 2025 at 08:16:05AM -0400, Sasha Levin wrote:

This is a note to let you know that I've just added the patch titled

PCI: pciehp: Ignore Link Down/Up caused by Secondary Bus Reset

to the 6.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: pci-pciehp-ignore-link-down-up-caused-by-secondary-b.patch and it can be found in the queue-6.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

Hi Sasha, thanks for selecting the above (which is 2af781a9edc4 upstream) as a 6.15 backport.

A small feature request, could you amend the stable tooling to cc people tagged as Reported-by and Tested-by? I think they're the ones most interested in seeing something backported.

Thanks!

Lukas

commit 161a7237de69f65ccfe68da318343f3719149480 Author: Lukas Wunner lukas@wunner.de Date: Thu Apr 10 17:27:12 2025 +0200

PCI: pciehp: Ignore Link Down/Up caused by Secondary Bus Reset

[ Upstream commit 2af781a9edc4ef5f6684c0710cc3542d9be48b31 ] When a Secondary Bus Reset is issued at a hotplug port, it causes a Data Link Layer State Changed event as a side effect. On hotplug ports using in-band presence detect, it additionally causes a Presence Detect Changed event. These spurious events should not result in teardown and re-enumeration of the device in the slot. Hence commit 2e35afaefe64 ("PCI: pciehp: Add reset_slot() method") masked the Presence Detect Changed Enable bit in the Slot Control register during a Secondary Bus Reset. Commit 06a8d89af551 ("PCI: pciehp: Disable link notification across slot reset") additionally masked the Data Link Layer State Changed Enable bit. However masking those bits only disables interrupt generation (PCIe r6.2 sec 6.7.3.1). The events are still visible in the Slot Status register and picked up by the IRQ handler if it runs during a Secondary Bus Reset. This can happen if the interrupt is shared or if an unmasked hotplug event occurs, e.g. Attention Button Pressed or Power Fault Detected. The likelihood of this happening used to be small, so it wasn't much of a problem in practice. That has changed with the recent introduction of bandwidth control in v6.13-rc1 with commit 665745f27487 ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller"): Bandwidth control shares the interrupt with PCIe hotplug. A Secondary Bus Reset causes a Link Bandwidth Notification, so the hotplug IRQ handler runs, picks up the masked events and tears down the device in the slot. As a result, Joel reports VFIO passthrough failure of a GPU, which Ilpo root-caused to the incorrect handling of masked hotplug events. Clearly, a more reliable way is needed to ignore spurious hotplug events. For Downstream Port Containment, a new ignore mechanism was introduced by commit a97396c6eb13 ("PCI: pciehp: Ignore Link Down/Up caused by DPC"). It has been working reliably for the past four years. Adapt it for Secondary Bus Resets. Introduce two helpers to annotate code sections which cause spurious link changes: pci_hp_ignore_link_change() and pci_hp_unignore_link_change() Use those helpers in lieu of masking interrupts in the Slot Control register. Introduce a helper to check whether such a code section is executing concurrently and if so, await it: pci_hp_spurious_link_change() Invoke the helper in the hotplug IRQ thread pciehp_ist(). Re-use the IRQ thread's existing code which ignores DPC-induced link changes unless the link is unexpectedly down after reset recovery or the device was replaced during the bus reset. That code block in pciehp_ist() was previously only executed if a Data Link Layer State Changed event has occurred. Additionally execute it for Presence Detect Changed events. That's necessary for compatibility with PCIe r1.0 hotplug ports because Data Link Layer State Changed didn't exist before PCIe r1.1. DPC was added with PCIe r3.1 and thus DPC-capable hotplug ports always support Data Link Layer State Changed events. But the same cannot be assumed for Secondary Bus Reset, which already existed in PCIe r1.0. Secondary Bus Reset is only one of many causes of spurious link changes. Others include runtime suspend to D3cold, firmware updates or FPGA reconfiguration. The new pci_hp_{,un}ignore_link_change() helpers may be used by all kinds of drivers to annotate such code sections, hence their declarations are publicly visible in <linux/pci.h>. A case in point is the Mellanox Ethernet driver which disables a firmware reset feature if the Ethernet card is attached to a hotplug port, see commit 3d7a3f2612d7 ("net/mlx5: Nack sync reset request when HotPlug is enabled"). Going forward, PCIe hotplug will be able to cope gracefully with all such use cases once the code sections are properly annotated. The new helpers internally use two bits in struct pci_dev's priv_flags as well as a wait_queue. This mirrors what was done for DPC by commit a97396c6eb13 ("PCI: pciehp: Ignore Link Down/Up caused by DPC"). That may be insufficient if spurious link changes are caused by multiple sources simultaneously. An example might be a Secondary Bus Reset issued by AER during FPGA reconfiguration. If this turns out to happen in real life, support for it can easily be added by replacing the PCI_LINK_CHANGING flag with an atomic_t counter incremented by pci_hp_ignore_link_change() and decremented by pci_hp_unignore_link_change(). Instead of awaiting a zero PCI_LINK_CHANGING flag, the pci_hp_spurious_link_change() helper would then simply await a zero counter. Fixes: 665745f27487 ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller") Reported-by: Joel Mathew Thomas proxy0@tutamail.com Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219765 Signed-off-by: Lukas Wunner lukas@wunner.de Signed-off-by: Bjorn Helgaas bhelgaas@google.com Tested-by: Joel Mathew Thomas proxy0@tutamail.com Reviewed-by: Kuppuswamy Sathyanarayanan sathyanarayanan.kuppuswamy@linux.intel.com Reviewed-by: Ilpo Järvinen ilpo.jarvinen@linux.intel.com Link: https://patch.msgid.link/d04deaf49d634a2edf42bf3c06ed81b4ca54d17b.1744298239... Signed-off-by: Sasha Levin sashal@kernel.org