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.

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>

2 files changed, 45 insertions, 0 deletions

diff --git a/include/net/devlink.h b/include/net/devlink.h
index 5f479227144d..cb839e0435a1 100644
--- a/include/net/devlink.h
+++ b/include/net/devlink.h
@@ -479,6 +479,10 @@ struct devlink_flash_notify {
  * @set: set parameter value, used for runtime and permanent
  *       configuration modes
  * @validate: validate input value is applicable (within value range, etc.)
+ * @get_default: get parameter default value, used for runtime and permanent
+ *               configuration modes
+ * @reset_default: reset parameter to default value, used for runtime and permanent
+ *                 configuration modes
  *
  * This struct should be used by the driver to fill the data for
  * a parameter it registers.
@@ -498,6 +502,12 @@ struct devlink_param {
 	int (*validate)(struct devlink *devlink, u32 id,
 			union devlink_param_value val,
 			struct netlink_ext_ack *extack);
+	int (*get_default)(struct devlink *devlink, u32 id,
+			   struct devlink_param_gset_ctx *ctx,
+			   struct netlink_ext_ack *extack);
+	int (*reset_default)(struct devlink *devlink, u32 id,
+			     enum devlink_param_cmode cmode,
+			     struct netlink_ext_ack *extack);
 };
 
 struct devlink_param_item {
@@ -509,6 +519,7 @@ struct devlink_param_item {
 							 * until reload.
 							 */
 	bool driverinit_value_new_valid;
+	union devlink_param_value driverinit_default;
 };
 
 enum devlink_param_generic_id {
@@ -630,6 +641,37 @@ enum devlink_param_generic_id {
 	.validate = _validate,						\
 }
 
+#define DEVLINK_PARAM_GENERIC_WITH_DEFAULTS(_id, _cmodes, _get, _set,	      \
+					    _validate, _get_default,	      \
+					    _reset_default)		      \
+{									      \
+	.id = DEVLINK_PARAM_GENERIC_ID_##_id,				      \
+	.name = DEVLINK_PARAM_GENERIC_##_id##_NAME,			      \
+	.type = DEVLINK_PARAM_GENERIC_##_id##_TYPE,			      \
+	.generic = true,						      \
+	.supported_cmodes = _cmodes,					      \
+	.get = _get,							      \
+	.set = _set,							      \
+	.validate = _validate,						      \
+	.get_default = _get_default,					      \
+	.reset_default = _reset_default,				      \
+}
+
+#define DEVLINK_PARAM_DRIVER_WITH_DEFAULTS(_id, _name, _type, _cmodes,	      \
+					   _get, _set, _validate,	      \
+					   _get_default, _reset_default)      \
+{									      \
+	.id = _id,							      \
+	.name = _name,							      \
+	.type = _type,							      \
+	.supported_cmodes = _cmodes,					      \
+	.get = _get,							      \
+	.set = _set,							      \
+	.validate = _validate,						      \
+	.get_default = _get_default,					      \
+	.reset_default = _reset_default,				      \
+}
+
 /* Identifier of board design */
 #define DEVLINK_INFO_VERSION_GENERIC_BOARD_ID	"board.id"
 /* Revision of board design */
diff --git a/include/uapi/linux/devlink.h b/include/uapi/linux/devlink.h
index 157f11d3fb72..e7d6b6d13470 100644
--- a/include/uapi/linux/devlink.h
+++ b/include/uapi/linux/devlink.h
@@ -639,6 +639,9 @@ enum devlink_attr {
 
 	DEVLINK_ATTR_HEALTH_REPORTER_BURST_PERIOD,	/* u64 */
 
+	DEVLINK_ATTR_PARAM_VALUE_DEFAULT,	/* dynamic */
+	DEVLINK_ATTR_PARAM_RESET_DEFAULT,	/* flag */
+
 	/* Add new attributes above here, update the spec in
 	 * Documentation/netlink/specs/devlink.yaml and re-generate
 	 * net/devlink/netlink_gen.c.