Re: [PATCH 1/2] PCI/CXL: Allow PM Init to complete on cxl_bus reset if ACS SV enabled

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Apr 28, 2026 at 08:24:34PM +0200, Fabio M. De Francesco wrote:
> CXL r4.0 sec 8.1.5.1 Implementation Note describes a scenario in which
> issuing a Secondary Bus Reset on a CXL Downstream Port leaves the
> Port Power Management Initialization Complete bit unset when the PCIe
> Access Control Service (ACS) Source Validation bit (SV) is enabled on
> the Downstream Port. The spec states that another SBR alone will not
> facilitate recovery and shows a software recovery sequence.
> 
> Implement the sequence by extending cxl_reset_bus_function() to save,
> clear, and restore ACS SV and Bus Master Enable (BME) on the Downstream
> Port around the SBR with the use of helpers.
> 
> The wait inside pci_bridge_secondary_bus_reset() covers the 100 ms
> referenced by the spec. The helpers return when ACS SV is not enabled on
> the Downstream Port.

When reposting, fix subject line of cover letter.

> + * cxl_reset_bus_function - SBR for a child of a CXL downstream port
> + * @dev: child device whose upstream bridge is a CXL downstream port
> + * @probe: if true, only check whether the reset is supported
> + *
> + * Issues an SBR on @dev's parent bus. Temporarily sets the CXL Port
> + * DVSEC Unmask SBR bit across the reset. When ACS Source Validation
> + * is enabled on the bridge, also temporarily clears Bus Master Enable
> + * and ACS Source Validation, per CXL r4.0 sec 8.1.5.1.

Use imperative mood consistently.  ("Issue ...", "Temporarily set
...").

> + * Return: 0 on success, -ENOTTY if the reset cannot be issued, or an
> + * errno from the reset path.
> + */
>  static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
>  {
>  	struct pci_dev *bridge;
>  	u16 dvsec, reg, val;
> +	u16 saved_cmd = 0, saved_acs_ctrl = 0;
>  	int rc;
>  
>  	bridge = pci_upstream_bridge(dev);
> @@ -4957,6 +5002,8 @@ static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
>  		return rc;
>  	}
>  
> +	cxl_disable_acs_sv_bme(bridge, &saved_cmd, &saved_acs_ctrl);
> +
>  	if (reg & PCI_DVSEC_CXL_PORT_CTL_UNMASK_SBR) {
>  		val = reg;
>  	} else {
> @@ -4971,6 +5018,8 @@ static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
>  		pci_write_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL,
>  				      reg);
>  
> +	cxl_restore_acs_sv_bme(bridge, saved_cmd, saved_acs_ctrl);
> +
>  	pci_dev_reset_iommu_done(dev);
>  	return rc;
>  }
> -- 
> 2.53.0
>