Re: [PATCH v3] PCI/portdrv: Allow probing even without child services

[Brian Norris <briannorris@chromium.org>]

Hi sashiko-bot and Bjorn,

On Fri, May 08, 2026 at 11:30:22PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Moving `pci_set_master()` after child device registration causes child drivers to probe with Bus Mastering disabled, risking permanently lost MSI/MSI-X interrupts.
> --
> 
> commit 9b851bf8faf271ab0b2daad51bfd51f89a1e87d4
> Author: Brian Norris <briannorris@chromium.org>
> 
> PCI/portdrv: Allow probing even without child services
> 
> This commit modifies the PCIe port driver to allow probe to succeed even
> if no child services are available. This ensures the port can still be
> configured for runtime PM and low power modes on systems that only have
> certain port services available or lack legacy INTx interrupts.
> 
> Link: https://lore.kernel.org/all/nyada24tqwlkzdceyoxbzitzygvp4elvj5oajnqdwb33xkcdwk@76vnrx45fsfd/
> 
> > --- a/drivers/pci/pcie/portdrv.c
> > +++ b/drivers/pci/pcie/portdrv.c
> > @@ -343,7 +343,6 @@ static int pcie_port_device_register(struct pci_dev *dev)
> >  	if (!capabilities)
> >  		return 0;
> >  
> > -	pci_set_master(dev);
> >  	/*
> >  	 * Initialize service irqs. Don't use service devices that
> >  	 * require interrupts if there is no way to generate them.
> 
> [ ... ]
> 
> >  	/* Allocate child services if any */
> > -	status = -ENODEV;
> > -	nr_service = 0;
> >  	for (i = 0; i < PCIE_PORT_DEVICE_MAXSERVICES; i++) {
> >  		int service = 1 << i;
> >  		if (!(capabilities & service))
> >  			continue;
> > -		if (!pcie_device_init(dev, service, irqs[i]))
> > -			nr_service++;
> > +		pcie_device_init(dev, service, irqs[i]);
> >  	}
> > -	if (!nr_service)
> > -		goto error_cleanup_irqs;
> >  
> > -	return 0;
> > +	pci_set_master(dev);
> 
> Does moving pci_set_master() after the pcie_device_init() loop risk dropping
> MSI/MSI-X interrupts during child driver probes?
> 
> The pcie_device_init() function calls device_register(), which synchronously
> invokes the probe routines of child service drivers like pciehp or aer.
> During probe, these child drivers configure the hardware and enable their
> interrupts.
> 
> Because PCIe MSI and MSI-X interrupts are memory write transactions, they
> require the Bus Master Enable (BME) bit to be set in the PCI Command Register.
> 
> If a hardware event occurs immediately after the child driver unmasks the
> interrupt but before pci_set_master(dev) is called, will the hardware attempt
> to send an MSI and drop the memory write because BME is 0?
> 
> Since PCIe interrupts are edge-triggered messages, could these dropped
> interrupts result in permanently missed hotplug events or cause hardware error
> handling regressions?

Ha, that's funny. It's all a great question I think, and that's pretty
much what I asked Bjorn here when he suggested moving this:

https://lore.kernel.org/all/aZfIfn9viQRj4uy4@google.com/

He seems to think it's not a problem:

https://lore.kernel.org/all/20260220164046.GA3528004@bhelgaas/

but I'm frankly not very convinced. I think it's perfectly possible that
a child service will try to enable MSI, the device will try to write,
and those writes will be dropped, producing a missing interrupt.

Bjorn, what do you think? Personally, I'd go back to something closer to
v2, where we enable mastering before initializing children. But maybe
you know better than this bot and me.

Frankly, this is all a distraction from the actual goal of my patch, so
I'd rather not make arbitrary changes to the ordering if we're not sure.

Brian

> > -error_cleanup_irqs:
> > -	pci_free_irq_vectors(dev);
> > -error_disable:
> > -	pci_disable_device(dev);
> > -	return status;
> > +	return 0;
> >  }
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260508133629.v3.1.I5fd5d83f518681b3949d8ab2f16ba8244fd3e774@changeid?part=1