RE: [PATCH v5 25/26] PCI/AER: Forward RCH downstream port-detected errors to the CXL.mem dev handler

[Dan Williams <dan.j.williams@intel.com>]

Terry Bowman wrote:
> From: Robert Richter <rrichter@amd.com>
> 
> 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
> Requster 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@amd.com>
> Signed-off-by: Terry Bowman <terry.bowman@amd.com>
> Signed-off-by: Robert Richter <rrichter@amd.com>
> Cc: "Oliver O'Halloran" <oohall@gmail.com>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-pci@vger.kernel.org
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> ---
>  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)"
> +	default y
> +	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;

I think this is ok for now, but I expect it will want to be something
like dev->is_cxl in the future where that is the cached result of:

        dev->is_cxl = !!pci_find_dvsec_capability(dev, PCI_DVSEC_VENDOR_ID_CXL,
                                                  CXL_DVSEC_PCIE_DEVICE);

> +
> +	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);

If I understand the RDPAS implementation note correctly, 

"Probe all CXL Downstream Ports and determine whether they have logged an
error in the CXL.io or CXL.cachemem status registers."

A VH topology would want to do something similar for root port error
events that get routed to an event collector. Now I do not think the VH
case needs to be solved in this patchset, but I believe this function
can be called cxl_rcec_handle_error(), and not need to be specific about
the topologies it is handling.

Other than that naming quibble, this patch looks good to me.