devlink: support default values for param-get and param-set

Support querying and resetting to default param values. Introduce two new devlink netlink attrs: DEVLINK_ATTR_PARAM_VALUE_DEFAULT and DEVLINK_ATTR_PARAM_RESET_DEFAULT. The former is used to contain an optional parameter value inside of the param_value nested attribute. The latter is used in param-set requests from userspace to indicate that the driver should reset the param to its default value. To implement this, two new functions are added to the devlink driver api: devlink_param::get_default() and devlink_param::reset_default(). These callbacks allow drivers to implement default param actions for runtime and permanent cmodes. For driverinit params, the core latches the last value set by a driver via devl_param_driverinit_value_set(), and uses that as the default value for a param. Because default parameter values are optional, it would be impossible to discern whether or not a param of type bool has default value of false or not provided if the default value is encoded using a netlink flag type. For this reason, when a DEVLINK_PARAM_TYPE_BOOL has an associated default value, the default value is encoded using a u8 type.

DZDaniel Zahka committed 11 months ago