Re: [PATCH v1 2/5] PCI/CXL: Probe the underlying bus reset in cxl_reset_bus_function()

[Bjorn Helgaas]

On Tue, Nov 03, 2026 at 10:33:43AM -0800, Nicolin Chen wrote:
> cxl_reset_bus_function() reports "supported" to a probe after checking only
> that the upstream bridge carries a CXL port DVSEC. The underlying bus reset
> can still be unavailable, e.g. on a bus shared with other devices, so both
> the reset_methods[] array and the reset_method sysfs node end up listing a
> "cxl_bus" that is guaranteed to fail with -ENOTTY when it is attempted.
> 
> Probe the underlying pci_dev_reset_slot_function() and then, if it is not
> applicable, pci_parent_bus_reset(). These are the same two checks that the
> actual reset runs, so a shared-bus CXL device no longer advertises a method
> that can never succeed.
> 
> Probing via pci_reset_bus_function() would not work: its cxl_sbr_masked()
> check rejects every CXL port with a masked SBR, while the do-reset path in
> this function unmasks the SBR before resetting. Such a port would wrongly
> probe as unsupported.
> 
> Move the port control register read above the probe branch as well, so that
> probe runs the same checks. That read asserts no reset, and a failure means
> the device cannot be reset this way, which is a probe-time answer.
> 
> Also pass an explicit PCI_RESET_DO_RESET at the do-reset call site, since
> probe is always false at that point.
> 
> Fixes: 53c49b6e6dd2e ("PCI/CXL: Add 'cxl_bus' reset method for devices below CXL Ports")
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>

Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>

> ---
>  drivers/pci/pci.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 38b680e7c3d60..e9b06b360850e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5075,13 +5075,21 @@ static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
>  	if (!dvsec)
>  		return -ENOTTY;
>  
> -	if (probe)
> -		return 0;
> -
>  	rc = pci_read_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL, &reg);
>  	if (rc)
>  		return -ENOTTY;
>  
> +	/*
> +	 * Do not probe via pci_reset_bus_function(), which would reject a
> +	 * masked SBR that the do-reset path below unmasks before resetting.
> +	 */
> +	if (probe) {
> +		rc = pci_dev_reset_slot_function(dev, PCI_RESET_PROBE);
> +		if (rc != -ENOTTY)
> +			return rc;
> +		return pci_parent_bus_reset(dev, PCI_RESET_PROBE);
> +	}
> +
>  	rc = pci_dev_reset_iommu_prepare(dev);
>  	if (rc) {
>  		pci_err(dev, "failed to stop IOMMU for a PCI reset: %d\n", rc);
> @@ -5096,7 +5104,7 @@ static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
>  				      val);
>  	}
>  
> -	rc = pci_reset_bus_function(dev, probe);
> +	rc = pci_reset_bus_function(dev, PCI_RESET_DO_RESET);
>  
>  	if (reg != val)
>  		pci_write_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL,
> -- 
> 2.43.0
>