RE: [PATCH v4 3/4] cxl: Fix sysfs export of qos_class for memdev

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

Dave Jiang wrote:
> Current implementation exports only to
> /sys/bus/cxl/devices/.../memN/qos_class. With both ram and pmem exposed,
> the second registered sysfs attribute is rejected as duplicate. It's not
> possible to create qos_class under the dev_groups via the driver due to
> the ram and pmem sysfs sub-directories already created by the device sysfs
> groups. Move the ram and pmem qos_class to the device sysfs groups and add
> a call to sysfs_update() after the perf data are validated so the
> qos_class can be visible. The end results should be
> /sys/bus/cxl/devices/.../memN/ram/qos_class and
> /sys/bus/cxl/devices/.../memN/pmem/qos_class.
> 
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
> v4:
> - Replace open code with sysfs_update_groups() helper. (Jonathan)
> ---
>  drivers/cxl/core/cdat.c   |  1 +
>  drivers/cxl/core/memdev.c | 66 +++++++++++++++++++++++++++++++++++++++
>  drivers/cxl/cxl.h         |  2 ++
>  drivers/cxl/mem.c         | 36 ---------------------
>  4 files changed, 69 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/cxl/core/cdat.c b/drivers/cxl/core/cdat.c
> index ecbd209ca70a..f5bebc7e7ccf 100644
> --- a/drivers/cxl/core/cdat.c
> +++ b/drivers/cxl/core/cdat.c
> @@ -382,6 +382,7 @@ void cxl_endpoint_parse_cdat(struct cxl_port *port)
>  
>  	cxl_memdev_set_qos_class(cxlds, dsmas_xa);
>  	cxl_qos_class_verify(cxlmd);
> +	cxl_memdev_update_attribute_groups(cxlmd);
>  }
>  EXPORT_SYMBOL_NS_GPL(cxl_endpoint_parse_cdat, CXL);
>  
> diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
> index dae8802ecdb0..c81f2cd4bcc6 100644
> --- a/drivers/cxl/core/memdev.c
> +++ b/drivers/cxl/core/memdev.c
> @@ -447,13 +447,41 @@ static struct attribute *cxl_memdev_attributes[] = {
>  	NULL,
>  };
>  
> +static ssize_t pmem_qos_class_show(struct device *dev,
> +				   struct device_attribute *attr, char *buf)
> +{
> +	struct cxl_memdev *cxlmd = to_cxl_memdev(dev);
> +	struct cxl_dev_state *cxlds = cxlmd->cxlds;
> +	struct cxl_memdev_state *mds = to_cxl_memdev_state(cxlds);
> +
> +	return sysfs_emit(buf, "%d\n", mds->pmem_perf.qos_class);
> +}
> +
> +static struct device_attribute dev_attr_pmem_qos_class =
> +	__ATTR(qos_class, 0444, pmem_qos_class_show, NULL);
> +
>  static struct attribute *cxl_memdev_pmem_attributes[] = {
>  	&dev_attr_pmem_size.attr,
> +	&dev_attr_pmem_qos_class.attr,
>  	NULL,
>  };
>  
> +static ssize_t ram_qos_class_show(struct device *dev,
> +				  struct device_attribute *attr, char *buf)
> +{
> +	struct cxl_memdev *cxlmd = to_cxl_memdev(dev);
> +	struct cxl_dev_state *cxlds = cxlmd->cxlds;
> +	struct cxl_memdev_state *mds = to_cxl_memdev_state(cxlds);
> +
> +	return sysfs_emit(buf, "%d\n", mds->ram_perf.qos_class);
> +}
> +
> +static struct device_attribute dev_attr_ram_qos_class =
> +	__ATTR(qos_class, 0444, ram_qos_class_show, NULL);
> +
>  static struct attribute *cxl_memdev_ram_attributes[] = {
>  	&dev_attr_ram_size.attr,
> +	&dev_attr_ram_qos_class.attr,
>  	NULL,
>  };
>  
> @@ -477,14 +505,42 @@ static struct attribute_group cxl_memdev_attribute_group = {
>  	.is_visible = cxl_memdev_visible,
>  };
>  
> +static umode_t cxl_ram_visible(struct kobject *kobj, struct attribute *a, int n)
> +{
> +	struct device *dev = kobj_to_dev(kobj);
> +	struct cxl_memdev *cxlmd = to_cxl_memdev(dev);
> +	struct cxl_memdev_state *mds = to_cxl_memdev_state(cxlmd->cxlds);
> +
> +	if (a == &dev_attr_ram_qos_class.attr)
> +		if (mds->ram_perf.qos_class == CXL_QOS_CLASS_INVALID)
> +			return 0;
> +
> +	return a->mode;
> +}
> +
>  static struct attribute_group cxl_memdev_ram_attribute_group = {
>  	.name = "ram",
>  	.attrs = cxl_memdev_ram_attributes,
> +	.is_visible = cxl_ram_visible,
>  };
>  
> +static umode_t cxl_pmem_visible(struct kobject *kobj, struct attribute *a, int n)
> +{
> +	struct device *dev = kobj_to_dev(kobj);
> +	struct cxl_memdev *cxlmd = to_cxl_memdev(dev);
> +	struct cxl_memdev_state *mds = to_cxl_memdev_state(cxlmd->cxlds);
> +
> +	if (a == &dev_attr_pmem_qos_class.attr)
> +		if (mds->pmem_perf.qos_class == CXL_QOS_CLASS_INVALID)
> +			return 0;
> +
> +	return a->mode;
> +}
> +
>  static struct attribute_group cxl_memdev_pmem_attribute_group = {
>  	.name = "pmem",
>  	.attrs = cxl_memdev_pmem_attributes,
> +	.is_visible = cxl_pmem_visible,
>  };
>  
>  static umode_t cxl_memdev_security_visible(struct kobject *kobj,
> @@ -519,6 +575,16 @@ static const struct attribute_group *cxl_memdev_attribute_groups[] = {
>  	NULL,
>  };
>  
> +void cxl_memdev_update_attribute_groups(struct cxl_memdev *cxlmd)
> +{
> +	int rc;
> +
> +	rc = sysfs_update_groups(&cxlmd->dev.kobj, cxl_memdev_attribute_groups);
> +	if (rc)
> +		dev_dbg(&cxlmd->dev, "Unable to update memdev attribute group.\n");
> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_memdev_update_attribute_groups, CXL);

So I appreciate that Jonathan pointed out this helper as a replacement
for the hand coded loop over all the attribute groups, but I don't
understand why all the groups need to be revisited when the groups to
update are known? It is also a red-flag to handle the result of a
__must_check helper with a silent dev_dbg() statement.

Given this qos-class support is optional, sysfs_update_groups(), with
its violent "tear it all down on failure" semantic, is too heavyweight.
The failure can not really be handled from the endpoint port code
because the side-effect tears down all groups even the ones registered
before the endpoint port driver loads.

Instead, just do something like this:

diff --git a/drivers/cxl/core/memdev.c b/drivers/cxl/core/memdev.c
index c81f2cd4bcc6..d4e259f3a7e9 100644
--- a/drivers/cxl/core/memdev.c
+++ b/drivers/cxl/core/memdev.c
@@ -575,15 +575,12 @@ static const struct attribute_group *cxl_memdev_attribute_groups[] = {
        NULL,
 };
 
-void cxl_memdev_update_attribute_groups(struct cxl_memdev *cxlmd)
+void cxl_memdev_update_perf(struct cxl_memdev *cxlmd)
 {
-       int rc;
-
-       rc = sysfs_update_groups(&cxlmd->dev.kobj, cxl_memdev_attribute_groups);
-       if (rc)
-               dev_dbg(&cxlmd->dev, "Unable to update memdev attribute group.\n");
+       sysfs_update_group(&cxlmd->dev.kobj, &cxl_memdev_ram_attribute_group);
+       sysfs_update_group(&cxlmd->dev.kobj, &cxl_memdev_pmem_attribute_group);
 }
-EXPORT_SYMBOL_NS_GPL(cxl_memdev_update_attribute_groups, CXL);
+EXPORT_SYMBOL_NS_GPL(cxl_memdev_update_perf, CXL);
 
 static const struct device_type cxl_memdev_type = {
        .name = "cxl_memdev",

...where failures are truly ignored.