Re: [PATCH v4 3/3] cxl/pci: Add sysfs attribute for CXL 1.1 device link status

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Apr 09, 2024 at 04:35:28PM +0900, Kobayashi,Daisuke wrote:
> Add sysfs attribute for CXL 1.1 device link status to the cxl pci device.
> 
> In CXL1.1, the link status of the device is included in the RCRB mapped to
> the memory mapped register area. Critically, that arrangement makes the link
> status and control registers invisible to existing PCI user tooling.

Idle thought: PCIe does define RCRB, even pre-CXL.  Maybe the PCI core
should be enhanced to comprehend RCRB directly?

> +static ssize_t rcd_link_status_show(struct device *dev,
> +				   struct device_attribute *attr, char *buf)
> +{
> +	struct cxl_port *port;
> +	struct cxl_dport *dport;
> +	struct device *parent = dev->parent;
> +	struct pci_dev *parent_pdev = to_pci_dev(parent);
> +
> +	port = cxl_pci_find_port(parent_pdev, &dport);
> +	if (!port)
> +		return -EINVAL;
> +
> +	return sysfs_emit(buf, "%x\n", dport->rcrb.rcd_lnkstatus);

Is it really what you want to capture PCI_EXP_LNKSTA once at
enumeration-time and expose that static value forever?  I assume
status bits can change over time, so I would naively expect that you
want the *current* value, not just a value from the distant past.

Bjorn