Re: [PATCH v10 13/15] PCI/AER: Forward RCH downstream port-detected errors to the CXL.mem dev handler

[Terry Bowman]

Hi Dan,

On 8/31/23 15:35, Dan Williams wrote:
> Terry Bowman wrote:
>> From: Robert Richter <rrichter@xxxxxxx>
>>
>> In Restricted CXL Device (RCD) mode a CXL device is exposed as an
>> RCiEP, but CXL downstream and upstream ports are not enumerated and
>> not visible in the PCIe hierarchy. [1] Protocol and link errors from
>> these non-enumerated ports are signaled as internal AER errors, either
>> Uncorrectable Internal Error (UIE) or Corrected Internal Errors (CIE)
>> via an RCEC.
>>
>> Restricted CXL host (RCH) downstream port-detected errors have the
>> Requester ID of the RCEC set in the RCEC's AER Error Source ID
>> register. A CXL handler must then inspect the error status in various
>> CXL registers residing in the dport's component register space (CXL
>> RAS capability) or the dport's RCRB (PCIe AER extended
>> capability). [2]
>>
>> Errors showing up in the RCEC's error handler must be handled and
>> connected to the CXL subsystem. Implement this by forwarding the error
>> to all CXL devices below the RCEC. Since the entire CXL device is
>> controlled only using PCIe Configuration Space of device 0, function
>> 0, only pass it there [3]. The error handling is limited to currently
>> supported devices with the Memory Device class code set (CXL Type 3
>> Device, PCI_CLASS_MEMORY_CXL, 502h), handle downstream port errors in
>> the device's cxl_pci driver. Support for other CXL Device Types
>> (e.g. a CXL.cache Device) can be added later.
>>
>> To handle downstream port errors in addition to errors directed to the
>> CXL endpoint device, a handler must also inspect the CXL RAS and PCIe
>> AER capabilities of the CXL downstream port the device is connected
>> to.
>>
>> Since CXL downstream port errors are signaled using internal errors,
>> the handler requires those errors to be unmasked. This is subject of a
>> follow-on patch.
>>
>> The reason for choosing this implementation is that the AER service
>> driver claims the RCEC device, but does not allow it to register a
>> custom specific handler to support CXL. Connecting the RCEC hard-wired
>> with a CXL handler does not work, as the CXL subsystem might not be
>> present all the time. The alternative to add an implementation to the
>> portdrv to allow the registration of a custom RCEC error handler isn't
>> worth doing it as CXL would be its only user. Instead, just check for
>> an CXL RCEC and pass it down to the connected CXL device's error
>> handler. With this approach the code can entirely be implemented in
>> the PCIe AER driver and is independent of the CXL subsystem. The CXL
>> driver only provides the handler.
>>
>> [1] CXL 3.0 spec: 9.11.8 CXL Devices Attached to an RCH
>> [2] CXL 3.0 spec, 12.2.1.1 RCH Downstream Port-detected Errors
>> [3] CXL 3.0 spec, 8.1.3 PCIe DVSEC for CXL Devices
>>
>> Co-developed-by: Terry Bowman <terry.bowman@xxxxxxx>
>> Signed-off-by: Terry Bowman <terry.bowman@xxxxxxx>
>> Signed-off-by: Robert Richter <rrichter@xxxxxxx>
>> Cc: "Oliver O'Halloran" <oohall@xxxxxxxxx>
>> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
>> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
>> Cc: linux-pci@xxxxxxxxxxxxxxx
>> Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
>> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
>> Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>
>> ---
>>  drivers/pci/pcie/Kconfig | 12 +++++
>>  drivers/pci/pcie/aer.c   | 96 +++++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 106 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/pci/pcie/Kconfig b/drivers/pci/pcie/Kconfig
>> index 228652a59f27..4f0e70fafe2d 100644
>> --- a/drivers/pci/pcie/Kconfig
>> +++ b/drivers/pci/pcie/Kconfig
>> @@ -49,6 +49,18 @@ config PCIEAER_INJECT
>>  	  gotten from:
>>  	     https://git.kernel.org/cgit/linux/kernel/git/gong.chen/aer-inject.git/
>>  
>> +config PCIEAER_CXL
>> +	bool "PCI Express CXL RAS support for Restricted Hosts (RCH)"
> 
> Why the "for Restricted Hosts (RCH)" clarification? I am seeing nothing
> that prevents this from working with RCECs on VH topologies.
> 

The same option can be used in VH mode. Will remove the RCH reference.

> 
>> +	default y
> 
> Minor, but I think "default PCIEAER" makes it slightly clearer that CXL
> error handling comes along for the ride with PCIE AER.
>

We found Kconfig entries do not typically list a dependancy and the default 
to be the same. We prefer to leave as 'default y'. If you want we can make 
your requested change. 

>> +	depends on PCIEAER && CXL_PCI
>> +	help
>> +	  Enables error handling of downstream ports of a CXL host
>> +	  that is operating in RCD mode (Restricted CXL Host, RCH).
>> +	  The downstream port reports AER errors to a given RCEC.
>> +	  Errors are handled by the CXL memory device driver.
>> +
>> +	  If unsure, say Y.
>> +
>>  #
>>  # PCI Express ECRC
>>  #
>> diff --git a/drivers/pci/pcie/aer.c b/drivers/pci/pcie/aer.c
>> index d3344fcf1f79..c354ca5e8f2b 100644
>> --- a/drivers/pci/pcie/aer.c
>> +++ b/drivers/pci/pcie/aer.c
>> @@ -946,14 +946,100 @@ static bool find_source_device(struct pci_dev *parent,
>>  	return true;
>>  }
>>  
>> +#ifdef CONFIG_PCIEAER_CXL
>> +
>> +static bool is_cxl_mem_dev(struct pci_dev *dev)
>> +{
>> +	/*
>> +	 * The capability, status, and control fields in Device 0,
>> +	 * Function 0 DVSEC control the CXL functionality of the
>> +	 * entire device (CXL 3.0, 8.1.3).
>> +	 */
>> +	if (dev->devfn != PCI_DEVFN(0, 0))
>> +		return false;
>> +
>> +	/*
>> +	 * CXL Memory Devices must have the 502h class code set (CXL
>> +	 * 3.0, 8.1.12.1).
>> +	 */
>> +	if ((dev->class >> 8) != PCI_CLASS_MEMORY_CXL)
>> +		return false;
> 
> Type-2 devices are going to support the same error flows but without
> advertising the CXL class code. Should this perhaps be something that
> CXL drivers can opt into by setting a flag in the pci_dev? It is already
> the case that the driver needs to be attached for the error handler to
> be found, so might as well allow the CXL AER handling to be opted-in by
> the driver as well.
> 

At the momment type-2 devices are unsupported and the drivers are not 
available. The absence of CXL class information in type-2 devices will present 
a challenge in identifying here. We would like to defer making change here 
and address this in a future a patchset. 

>> +
>> +	return true;
>> +}
>> +
>> +static bool cxl_error_is_native(struct pci_dev *dev)
>> +{
>> +	struct pci_host_bridge *host = pci_find_host_bridge(dev->bus);
>> +
>> +	if (pcie_ports_native)
>> +		return true;
>> +
>> +	return host->native_aer && host->native_cxl_error;
>> +}
>> +
>> +static bool is_internal_error(struct aer_err_info *info)
>> +{
>> +	if (info->severity == AER_CORRECTABLE)
>> +		return info->status & PCI_ERR_COR_INTERNAL;
>> +
>> +	return info->status & PCI_ERR_UNC_INTN;
>> +}
>> +
>> +static int cxl_rch_handle_error_iter(struct pci_dev *dev, void *data)
>> +{
>> +	struct aer_err_info *info = (struct aer_err_info *)data;
>> +	const struct pci_error_handlers *err_handler;
>> +
>> +	if (!is_cxl_mem_dev(dev) || !cxl_error_is_native(dev))
>> +		return 0;
>> +
>> +	/* protect dev->driver */
>> +	device_lock(&dev->dev);
>> +
>> +	err_handler = dev->driver ? dev->driver->err_handler : NULL;
>> +	if (!err_handler)
>> +		goto out;
>> +
>> +	if (info->severity == AER_CORRECTABLE) {
>> +		if (err_handler->cor_error_detected)
>> +			err_handler->cor_error_detected(dev);
>> +	} else if (err_handler->error_detected) {
>> +		if (info->severity == AER_NONFATAL)
>> +			err_handler->error_detected(dev, pci_channel_io_normal);
>> +		else if (info->severity == AER_FATAL)
>> +			err_handler->error_detected(dev, pci_channel_io_frozen);
>> +	}
>> +out:
>> +	device_unlock(&dev->dev);
>> +	return 0;
>> +}
>> +
>> +static void cxl_rch_handle_error(struct pci_dev *dev, struct aer_err_info *info)
>> +{
>> +	/*
>> +	 * Internal errors of an RCEC indicate an AER error in an
>> +	 * RCH's downstream port. Check and handle them in the CXL.mem
>> +	 * device driver.
>> +	 */
>> +	if (pci_pcie_type(dev) == PCI_EXP_TYPE_RC_EC &&
>> +	    is_internal_error(info))
>> +		pcie_walk_rcec(dev, cxl_rch_handle_error_iter, info);
> 
> This would seem to work generically for RCEC reported errors in a VH
> topology, so I think the "_rch" distinction can be dropped.
> 

The pcie_walk_rcec() filters on PCI_EXP_TYPE_RC_END devices. As a result this 
iterator will not apply to VH mode devices (PCI_EXP_TYPE_ENDPOINT, PCI_EXP_TYPE_ROOT_PORT,
PCI_EXP_TYPE_DOWNSTREAM). This series is focused on RCH mode. VH mode port error 
handling will be addressed in future patchset. 

Regards,
Terry

>> +}
>> +
>> +#else
>> +static inline void cxl_rch_handle_error(struct pci_dev *dev,
>> +					struct aer_err_info *info) { }
>> +#endif
>> +
>>  /**
>> - * handle_error_source - handle logging error into an event log
>> + * pci_aer_handle_error - handle logging error into an event log
>>   * @dev: pointer to pci_dev data structure of error source device
>>   * @info: comprehensive error information
>>   *
>>   * Invoked when an error being detected by Root Port.
>>   */
>> -static void handle_error_source(struct pci_dev *dev, struct aer_err_info *info)
>> +static void pci_aer_handle_error(struct pci_dev *dev, struct aer_err_info *info)
>>  {
>>  	int aer = dev->aer_cap;
>>  
>> @@ -977,6 +1063,12 @@ static void handle_error_source(struct pci_dev *dev, struct aer_err_info *info)
>>  		pcie_do_recovery(dev, pci_channel_io_normal, aer_root_reset);
>>  	else if (info->severity == AER_FATAL)
>>  		pcie_do_recovery(dev, pci_channel_io_frozen, aer_root_reset);
>> +}
>> +
>> +static void handle_error_source(struct pci_dev *dev, struct aer_err_info *info)
>> +{
>> +	cxl_rch_handle_error(dev, info);
>> +	pci_aer_handle_error(dev, info);
>>  	pci_dev_put(dev);
>>  }
>>  
>> -- 
>> 2.34.1
>>
> 
>