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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. Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com> Link: https://patch.msgid.link/20251119025038.651131-4-daniel.zahka@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>