Re: [PATCH v12 23/25] CXL/PCI: Introduce CXL uncorrectable protocol error recovery

[Dave Jiang <dave.jiang@intel.com>]

On 9/30/25 7:38 AM, Bowman, Terry wrote:
> 
> 
> On 9/29/2025 7:26 PM, Dave Jiang wrote:
>>
>> On 9/25/25 3:34 PM, Terry Bowman wrote:
>>> Populate the cxl_do_recovery() function with uncorrectable protocol error (UCE)
>>> handling. Follow similar design as found in PCIe error driver,
>>> pcie_do_recovery(). One difference is cxl_do_recovery() will treat all UCEs
>>> as fatal with a kernel panic. This is to prevent corruption on CXL memory.
>>>
>>> Introduce cxl_walk_port(). Make this analogous to pci_walk_bridge() but walking
>>> CXL ports instead. This will iterate through the CXL topology from the
>>> erroring device through the downstream CXL Ports and Endpoints.
>>>
>>> Export pci_aer_clear_fatal_status() for CXL to use if a UCE is not found.
>>>
>>> Signed-off-by: Terry Bowman <terry.bowman@amd.com>
>>>
>>> ---
>>>
>>> Changes in v11->v12:
>>> - Cleaned up port discovery in cxl_do_recovery() (Dave)
>>> - Added PCI_EXP_TYPE_RC_END to type check in cxl_report_error_detected()
>>>
>>> Changes in v10->v11:
>>> - pci_ers_merge_results() - Move to earlier patch
>>> ---
>>>  drivers/cxl/core/ras.c | 111 +++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 111 insertions(+)
>>>
>>> diff --git a/drivers/cxl/core/ras.c b/drivers/cxl/core/ras.c
>>> index 7e8d63c32d72..45f92defca64 100644
>>> --- a/drivers/cxl/core/ras.c
>>> +++ b/drivers/cxl/core/ras.c
>>> @@ -443,8 +443,119 @@ void cxl_endpoint_port_init_ras(struct cxl_port *ep)
>>>  }
>>>  EXPORT_SYMBOL_NS_GPL(cxl_endpoint_port_init_ras, "CXL");
>>>  
>>> +static int cxl_report_error_detected(struct device *dev, void *data)
>>> +{
>>> +	struct pci_dev *pdev = to_pci_dev(dev);
>>> +	pci_ers_result_t vote, *result = data;
>>> +
>>> +	guard(device)(dev);
>>> +
>>> +	if ((pci_pcie_type(pdev) == PCI_EXP_TYPE_ENDPOINT) ||
>>> +	    (pci_pcie_type(pdev) == PCI_EXP_TYPE_RC_END)) {
>>> +		if (!cxl_pci_drv_bound(pdev))
>>> +			return 0;
>>> +
>>> +		vote = cxl_error_detected(dev);
>>> +	} else {
>>> +		vote = cxl_port_error_detected(dev);
>>> +	}
>>> +
>>> +	*result = pci_ers_merge_result(*result, vote);
>>> +
>>> +	return 0;
>>> +}
>>> +
>>> +static int match_port_by_parent_dport(struct device *dev, const void *dport_dev)
>>> +{
>>> +	struct cxl_port *port;
>>> +
>>> +	if (!is_cxl_port(dev))
>>> +		return 0;
>>> +
>>> +	port = to_cxl_port(dev);
>>> +
>>> +	return port->parent_dport->dport_dev == dport_dev;
>>> +}
>>> +
>>> +static void cxl_walk_port(struct device *port_dev,
>>> +			  int (*cb)(struct device *, void *),
>>> +			  void *userdata)
>>> +{
>>> +	struct cxl_dport *dport = NULL;
>>> +	struct cxl_port *port;
>>> +	unsigned long index;
>>> +
>>> +	if (!port_dev)
>>> +		return;
>>> +
>>> +	port = to_cxl_port(port_dev);
>>> +	if (port->uport_dev && dev_is_pci(port->uport_dev))
>>> +		cb(port->uport_dev, userdata);
>> Could use some comments on what is being walked. Also an explanation of what is happening here would be good.
> Ok
>> If this is an endpoint port, this would be the PCI endpoint device.
>> If it's a switch port, then this is the upstream port.
>> If it's a root port, this is skipped.
>>
>>> +
>>> +	xa_for_each(&port->dports, index, dport)
>>> +	{
>>> +		struct device *child_port_dev __free(put_device) =
>>> +			bus_find_device(&cxl_bus_type, &port->dev, dport->dport_dev,
>>> +					match_port_by_parent_dport);
>>> +
>>> +		cb(dport->dport_dev, userdata);
>> This is going through all the downstream ports
>>> +
>>> +		cxl_walk_port(child_port_dev, cxl_report_error_detected, userdata);
>>> +	}
>>> +
>>> +	if (is_cxl_endpoint(port))
>>> +		cb(port->uport_dev->parent, userdata);
>> And this is the downstream parent port of the endpoint device
>>
>> Why not move this before the xa_for_each() and return early? endpoint ports don't have dports, no need to even try to run that block above.
> Sure, I'll change that.
>> So in the current implementation,
>> 1. Endpoint. It checks the device, and then it checks the downstream parent port for errors. Is checking the parent dport necessary?
>> 2. Switch. It checks the upstream port, then it checks all the downstream ports for errors.
>> 3. Root port. It checks all the downstream ports for errors.
>> Is this the correct understanding of what this function does?
> 
> Yes. The ordering is different as you pointed out. I can move the endpoint 
> check earlier with an early return. 

As the endpoint, what is the reason the check the parent dport? Pardon my ignorance.

>>> +}
>>> +
>>>  static void cxl_do_recovery(struct device *dev)
>>>  {
>>> +	pci_ers_result_t status = PCI_ERS_RESULT_CAN_RECOVER;
>>> +	struct pci_dev *pdev = to_pci_dev(dev);
>>> +	struct cxl_port *port = NULL;
>>> +
>>> +	if ((pci_pcie_type(pdev) == PCI_EXP_TYPE_ROOT_PORT) ||
>>> +	    (pci_pcie_type(pdev) == PCI_EXP_TYPE_DOWNSTREAM)) {
>>> +		struct cxl_dport *dport;
>>> +		struct cxl_port *rp_port __free(put_cxl_port) = find_cxl_port(&pdev->dev, &dport);
>>> +
>>> +		port = rp_port;
>>> +
>>> +	} else	if (pci_pcie_type(pdev) == PCI_EXP_TYPE_UPSTREAM) {
>>> +		struct cxl_port *us_port __free(put_cxl_port) = find_cxl_port_by_uport(&pdev->dev);
>>> +
>>> +		port = us_port;
>>> +
>>> +	} else	if ((pci_pcie_type(pdev) == PCI_EXP_TYPE_ENDPOINT) ||
>>> +		    (pci_pcie_type(pdev) == PCI_EXP_TYPE_RC_END)) {
>>> +		struct cxl_dev_state *cxlds;
>>> +
>>> +		if (!cxl_pci_drv_bound(pdev))
>>> +			return;
>> Need to have the pci dev lock before checking driver bound.
>> DJ
> 
> Ok, I'll try to add that into cxl_pci_drv_bound(). Terry

Do you need the lock beyond just checking the driver data? Maybe do it outside cxl_pci_drv_bound(). I would have an assert in the function though to ensure lock is held when calling this function.

DJ
>>> +
>>> +		cxlds = pci_get_drvdata(pdev);
>>> +		port = cxlds->cxlmd->endpoint;
>>> +	}
>>> +
>>> +	if (!port) {
>>> +		dev_err(dev, "Failed to find the CXL device\n");
>>> +		return;
>>> +	}
>>> +
>>> +	cxl_walk_port(&port->dev, cxl_report_error_detected, &status);
>>> +	if (status == PCI_ERS_RESULT_PANIC)
>>> +		panic("CXL cachemem error.");
>>> +
>>> +	/*
>>> +	 * If we have native control of AER, clear error status in the device
>>> +	 * that detected the error.  If the platform retained control of AER,
>>> +	 * it is responsible for clearing this status.  In that case, the
>>> +	 * signaling device may not even be visible to the OS.
>>> +	 */
>>> +	if (cxl_error_is_native(pdev)) {
>>> +		pcie_clear_device_status(pdev);
>>> +		pci_aer_clear_nonfatal_status(pdev);
>>> +		pci_aer_clear_fatal_status(pdev);
>>> +	}
>>>  }
>>>  
>>>  static void cxl_handle_cor_ras(struct device *dev, u64 serial, void __iomem *ras_base)
> 
>