Re: [PATCH v2] PCI/portdrv: Allow probing even without child services

[Brian Norris <briannorris@chromium.org>]

Hi,

On Thu, Feb 19, 2026 at 04:25:14PM -0600, Bjorn Helgaas wrote:
> On Mon, Feb 09, 2026 at 05:15:35PM -0800, Brian Norris wrote:
> > The PCIe port driver fails to probe if it finds no child services,
> > presumably under the assumption that the driver is not useful in that
> > case. However, the driver *can* still be useful for power management
> > support -- namely, it still configures the port for runtime PM / D3,
> > which may be important for allowing a bridge to enter low power modes.
> > 
> > Thus, allow probe to succeed even if no IRQs and no child services are
> > available. This also mirrors existing behavior for ports that have no
> > PCIe capabilities, where we'd also probe successfully.
> > 
> > This change is a bit more important after commit f5cd8a929c82 ("PCI:
> > dwc: Remove MSI/MSIX capability for Root Port if iMSI-RX is used as MSI
> > controller"), because it's common for some DWC-based systems to:
> > 
> > 1. have only have the "aer" and "pcie_pme" port services available and
> > 2. not define legacy INTx interrupts properly in their device tree.
> > 
> > After commit f5cd8a929c82, such systems may fail
> > pcie_init_service_irqs() and so exit with -ENODEV.
> > 
> > Link: https://lore.kernel.org/all/nyada24tqwlkzdceyoxbzitzygvp4elvj5oajnqdwb33xkcdwk@76vnrx45fsfd/
> > Signed-off-by: Brian Norris <briannorris@chromium.org>
> > ---
> > 
> > Changes in v2:
> >  * clear master when we have no child services
> > 
> >  drivers/pci/pcie/portdrv.c | 21 +++++++--------------
> >  1 file changed, 7 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/pci/pcie/portdrv.c b/drivers/pci/pcie/portdrv.c
> > index 88af0dacf351..19b08f3653ee 100644
> > --- a/drivers/pci/pcie/portdrv.c
> > +++ b/drivers/pci/pcie/portdrv.c
> > @@ -330,7 +330,7 @@ static int pcie_device_init(struct pci_dev *pdev, int service, int irq)
> >   */
> >  static int pcie_port_device_register(struct pci_dev *dev)
> >  {
> > -	int status, capabilities, i, nr_service;
> > +	int status, capabilities, i;
> >  	int irqs[PCIE_PORT_DEVICE_MAXSERVICES];
> >  
> >  	/* Enable PCI Express port device */
> > @@ -355,29 +355,22 @@ static int pcie_port_device_register(struct pci_dev *dev)
> >  	if (status) {
> >  		capabilities &= PCIE_PORT_SERVICE_HP;
> >  		if (!capabilities)
> > -			goto error_disable;
> > +			goto out;
> >  	}
> >  
> >  	/* 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;
> >  
> > +out:
> > +	/* With no child services, we shouldn't need bus mastering. */
> > +	if (!capabilities)
> > +		pci_clear_master(dev);
> 
> I'm curious about this part because we pci_set_master()
> unconditionally just above:
> 
>   pci_set_master(dev);
>   pcie_init_service_irqs(dev, irqs, capabilities);
>   for (i = 0; ...; i++)
>     pcie_device_init(dev, service, irqs[i]);
>   if (!capabilities)
>     pci_clear_master(dev);
> 
> Bus mastering on a bridge must be enabled for DMA from downstream
> devices to work, but I think that's done by pci_enable_bridge() when a
> driver calls pci_enable_device() for an endpoint.
> 
> I assume the reason we call pci_set_master() here is so MSI/MSI-X from
> the bridge will work, even if there is no downstream device.
> 
> I don't think either pcie_init_service_irqs() or pcie_device_init()
> requires bus mastering, so I don't know why we enable it here.  It
> seems like we should do it when we set up MSI/MSI-X interrupts.

I'm no expert here, but by code inspection, pcie_init_service_irqs() may
call pci_msi_set_enable() which sets PCI_MSI_FLAGS_ENABLE. Could that
confuse a device to see MSI enabled but bus mastering disabled?

Brian

> If we want to do it in pcie_port_device_register() (instead of in
> service driver when they set up an interrupt), maybe we should drop
> the initial pci_set_master() and do it conditionally, e.g.,
> 
>   if (capabilities)
>     pci_set_master(dev);