Re: [PATCH v7 2/2] cxl/pci: Add sysfs attribute for CXL 1.1 device link status

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Fri, 10 May 2024 16:37:10 +0900
"Kobayashi,Daisuke" <kobayashi.da-06@fujitsu.com> 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.
> 
> Export those registers via sysfs with the expectation that PCI user
> tooling will alternatively look for these sysfs files when attempting to
> access to these CXL 1.1 endpoints registers.

The lspci support raced with this series and has landed.

https://github.com/pciutils/pciutils/commit/49efa87fcce4f7d5b351238668ae1d4491802b88

A few follow up comments on suggestion to cache the offset, not the register value
(which might change over time)
> 
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: "Kobayashi,Daisuke" <kobayashi.da-06@fujitsu.com>
> ---
>  drivers/cxl/pci.c | 101 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 101 insertions(+)
> 
> diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> index 2ff361e756d6..c10797adde2c 100644
> --- a/drivers/cxl/pci.c
> +++ b/drivers/cxl/pci.c
> @@ -786,6 +786,106 @@ static int cxl_event_config(struct pci_host_bridge *host_bridge,
>  	return 0;
>  }
>  
> +static ssize_t rcd_link_cap_show(struct device *dev,
> +				   struct device_attribute *attr, char *buf)
> +{
> +	struct cxl_dev_state *cxlds = dev_get_drvdata(dev);
> +	struct cxl_memdev *cxlmd = cxlds->cxlmd;
> +	struct device *endpoint_parent;
> +	struct cxl_dport *dport;
> +	struct cxl_port *port;
> +
> +	port = cxl_mem_find_port(cxlmd, &dport);
> +	if (!port)
> +		return -EINVAL;
> +
> +	endpoint_parent = port->uport_dev;
> +	if (!endpoint_parent)
> +		return -ENXIO;
> +
> +	guard(device)(endpoint_parent);
> +	if (!endpoint_parent->driver)
> +		return -ENXIO;
> +
> +	return sysfs_emit(buf, "%x\n", dport->rcrb.rcd_lnkcap);

If you follow the suggestion of caching the offset, not the register
content then, these are all very similar. Define one common
function which takes a u8 offset and call that from each of the
functions.

If you stick to separate caches (and ensure they are up to date),
then you can add a utility function to take the dev here and
get you to the rcrb structure.  Then you can call that and only
have the final line in each instances of this code.

> +}
> +static DEVICE_ATTR_RO(rcd_link_cap);
> +
> +static ssize_t rcd_link_ctrl_show(struct device *dev,
> +				   struct device_attribute *attr, char *buf)
> +{
> +	struct cxl_dev_state *cxlds = dev_get_drvdata(dev);
> +	struct cxl_memdev *cxlmd = cxlds->cxlmd;
> +	struct device *endpoint_parent;
> +	struct cxl_dport *dport;
> +	struct cxl_port *port;
> +
> +	port = cxl_mem_find_port(cxlmd, &dport);
> +	if (!port)
> +		return -EINVAL;
> +
> +	endpoint_parent = port->uport_dev;
> +	if (!endpoint_parent)
> +		return -ENXIO;
> +
> +	guard(device)(endpoint_parent);
> +	if (!endpoint_parent->driver)
> +		return -ENXIO;
> +
> +	return sysfs_emit(buf, "%x\n", dport->rcrb.rcd_lnkctrl);
> +}
> +static DEVICE_ATTR_RO(rcd_link_ctrl);
> +
> +static ssize_t rcd_link_status_show(struct device *dev,
> +				   struct device_attribute *attr, char *buf)
> +{
> +	struct cxl_dev_state *cxlds = dev_get_drvdata(dev);
> +	struct cxl_memdev *cxlmd = cxlds->cxlmd;
> +	struct device *endpoint_parent;
> +	struct cxl_dport *dport;
> +	struct cxl_port *port;
> +
> +	port = cxl_mem_find_port(cxlmd, &dport);
> +	if (!port)
> +		return -EINVAL;
> +
> +	endpoint_parent = port->uport_dev;
> +	if (!endpoint_parent)
> +		return -ENXIO;
> +
> +	guard(device)(endpoint_parent);
> +	if (!endpoint_parent->driver)
> +		return -ENXIO;
> +
> +	return sysfs_emit(buf, "%x\n", dport->rcrb.rcd_lnkstatus);
> +}
> +static DEVICE_ATTR_RO(rcd_link_status);
> +
> +static struct attribute *cxl_rcd_attrs[] = {
> +		&dev_attr_rcd_link_cap.attr,
> +		&dev_attr_rcd_link_ctrl.attr,
> +		&dev_attr_rcd_link_status.attr,
> +		NULL
> +};
> +
> +static umode_t cxl_rcd_visible(struct kobject *kobj,
> +					  struct attribute *a, int n)
> +{
> +	struct device *dev = kobj_to_dev(kobj);
> +	struct pci_dev *pdev = to_pci_dev(dev);
> +
> +	if (is_cxl_restricted(pdev))
> +		return a->mode;
> +
> +	return 0;
> +}
> +
> +static struct attribute_group cxl_rcd_group = {
> +		.attrs = cxl_rcd_attrs,
> +		.is_visible = cxl_rcd_visible,
> +};
> +__ATTRIBUTE_GROUPS(cxl_rcd);
> +
>  static int cxl_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>  {
>  	struct pci_host_bridge *host_bridge = pci_find_host_bridge(pdev->bus);
> @@ -969,6 +1069,7 @@ static struct pci_driver cxl_pci_driver = {
>  	.id_table		= cxl_mem_pci_tbl,
>  	.probe			= cxl_pci_probe,
>  	.err_handler		= &cxl_error_handlers,
> +	.dev_groups		= cxl_rcd_groups,
>  	.driver	= {
>  		.probe_type	= PROBE_PREFER_ASYNCHRONOUS,
>  	},