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 <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/netlink/specs/devlink.yaml

···
         name: health-reporter-burst-period
         type: u64
         doc: Time (in msec) for recoveries before starting the grace period.
+
+      # TODO: fill in the attributes in between
+
+      -
+        name: param-reset-default
+        type: flag
+        doc: Request restoring parameter to its default value.
+        value: 183
   -
     name: dl-dev-stats
     subset-of: devlink
···
             - param-type
             # param-value-data is missing here as the type is variable
             - param-value-cmode
+            - param-reset-default
 
     -
       name: region-get

Documentation/networking/devlink/devlink-params.rst

···
 support reloading via the ``devlink-reload`` command. This command will
 request a reload of the device driver.
 
+Default parameter values
+=========================
+
+Drivers may optionally export default values for parameters of cmode
+``runtime`` and ``permanent``. For ``driverinit`` parameters, the last
+value set by the driver will be used as the default value. Drivers can
+also support resetting params with cmode ``runtime`` and ``permanent``
+to their default values. Resetting ``driverinit`` params is supported
+by devlink core without additional driver support needed.
+
 .. _devlink_params_generic:
 
 Generic configuration parameters

include/net/devlink.h

···
  * @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.
···
 	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 {
···
 							 * until reload.
 							 */
 	bool driverinit_value_new_valid;
+	union devlink_param_value driverinit_default;
 };
 
 enum devlink_param_generic_id {
···
 	.get = _get,							\
 	.set = _set,							\
 	.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 */

include/uapi/linux/devlink.h

···
 
 	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.

net/devlink/netlink_gen.c

···
 };
 
 /* DEVLINK_CMD_PARAM_SET - do */
-static const struct nla_policy devlink_param_set_nl_policy[DEVLINK_ATTR_PARAM_VALUE_CMODE + 1] = {
+static const struct nla_policy devlink_param_set_nl_policy[DEVLINK_ATTR_PARAM_RESET_DEFAULT + 1] = {
 	[DEVLINK_ATTR_BUS_NAME] = { .type = NLA_NUL_STRING, },
 	[DEVLINK_ATTR_DEV_NAME] = { .type = NLA_NUL_STRING, },
 	[DEVLINK_ATTR_PARAM_NAME] = { .type = NLA_NUL_STRING, },
 	[DEVLINK_ATTR_PARAM_TYPE] = NLA_POLICY_VALIDATE_FN(NLA_U8, &devlink_attr_param_type_validate),
 	[DEVLINK_ATTR_PARAM_VALUE_CMODE] = NLA_POLICY_MAX(NLA_U8, 2),
+	[DEVLINK_ATTR_PARAM_RESET_DEFAULT] = { .type = NLA_FLAG, },
 };
 
 /* DEVLINK_CMD_REGION_GET - do */
···
 		.doit		= devlink_nl_param_set_doit,
 		.post_doit	= devlink_nl_post_doit,
 		.policy		= devlink_param_set_nl_policy,
-		.maxattr	= DEVLINK_ATTR_PARAM_VALUE_CMODE,
+		.maxattr	= DEVLINK_ATTR_PARAM_RESET_DEFAULT,
 		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,
 	},
 	{

net/devlink/param.c

···
 	return param->set(devlink, param->id, ctx, extack);
 }
 
+static int devlink_param_get_default(struct devlink *devlink,
+				     const struct devlink_param *param,
+				     struct devlink_param_gset_ctx *ctx,
+				     struct netlink_ext_ack *extack)
+{
+	if (!param->get_default)
+		return -EOPNOTSUPP;
+
+	return param->get_default(devlink, param->id, ctx, extack);
+}
+
+static int devlink_param_reset_default(struct devlink *devlink,
+				       const struct devlink_param *param,
+				       enum devlink_param_cmode cmode,
+				       struct netlink_ext_ack *extack)
+{
+	if (!param->reset_default)
+		return -EOPNOTSUPP;
+
+	return param->reset_default(devlink, param->id, cmode, extack);
+}
+
 static int
 devlink_nl_param_value_put(struct sk_buff *msg, enum devlink_param_type type,
-			   int nla_type, union devlink_param_value val)
+			   int nla_type, union devlink_param_value val,
+			   bool flag_as_u8)
 {
 	switch (type) {
 	case DEVLINK_PARAM_TYPE_U8:
···
 			return -EMSGSIZE;
 		break;
 	case DEVLINK_PARAM_TYPE_BOOL:
-		if (val.vbool && nla_put_flag(msg, nla_type))
-			return -EMSGSIZE;
+		/* default values of type bool are encoded with u8, so that
+		 * false can be distinguished from not present
+		 */
+		if (flag_as_u8) {
+			if (nla_put_u8(msg, nla_type, val.vbool))
+				return -EMSGSIZE;
+		} else {
+			if (val.vbool && nla_put_flag(msg, nla_type))
+				return -EMSGSIZE;
+		}
 		break;
 	}
 	return 0;
···
 devlink_nl_param_value_fill_one(struct sk_buff *msg,
 				enum devlink_param_type type,
 				enum devlink_param_cmode cmode,
-				union devlink_param_value val)
+				union devlink_param_value val,
+				union devlink_param_value default_val,
+				bool has_default)
 {
 	struct nlattr *param_value_attr;
 	int err = -EMSGSIZE;
···
 		goto value_nest_cancel;
 
 	err = devlink_nl_param_value_put(msg, type,
-					 DEVLINK_ATTR_PARAM_VALUE_DATA, val);
+					 DEVLINK_ATTR_PARAM_VALUE_DATA,
+					 val, false);
 	if (err)
 		goto value_nest_cancel;
+
+	if (has_default) {
+		err = devlink_nl_param_value_put(msg, type,
+						 DEVLINK_ATTR_PARAM_VALUE_DEFAULT,
+						 default_val, true);
+		if (err)
+			goto value_nest_cancel;
+	}
 
 	nla_nest_end(msg, param_value_attr);
 	return 0;
···
 				 u32 portid, u32 seq, int flags,
 				 struct netlink_ext_ack *extack)
 {
+	union devlink_param_value default_value[DEVLINK_PARAM_CMODE_MAX + 1];
 	union devlink_param_value param_value[DEVLINK_PARAM_CMODE_MAX + 1];
+	bool default_value_set[DEVLINK_PARAM_CMODE_MAX + 1] = {};
 	bool param_value_set[DEVLINK_PARAM_CMODE_MAX + 1] = {};
 	const struct devlink_param *param = param_item->param;
 	struct devlink_param_gset_ctx ctx;
···
 				param_value[i] = param_item->driverinit_value;
 			else
 				return -EOPNOTSUPP;
+
+			if (param_item->driverinit_value_valid) {
+				default_value[i] = param_item->driverinit_default;
+				default_value_set[i] = true;
+			}
 		} else {
 			ctx.cmode = i;
 			err = devlink_param_get(devlink, param, &ctx, extack);
 			if (err)
 				return err;
 			param_value[i] = ctx.val;
+
+			err = devlink_param_get_default(devlink, param, &ctx,
+							extack);
+			if (!err) {
+				default_value[i] = ctx.val;
+				default_value_set[i] = true;
+			} else if (err != -EOPNOTSUPP) {
+				return err;
+			}
 		}
 		param_value_set[i] = true;
 	}
···
 		if (!param_value_set[i])
 			continue;
 		err = devlink_nl_param_value_fill_one(msg, param->type,
-						      i, param_value[i]);
+						      i, param_value[i],
+						      default_value[i],
+						      default_value_set[i]);
 		if (err)
 			goto values_list_nest_cancel;
 	}
···
 	struct devlink_param_item *param_item;
 	const struct devlink_param *param;
 	union devlink_param_value value;
+	bool reset_default;
 	int err = 0;
 
 	param_item = devlink_param_get_from_info(params, info);
···
 		return err;
 	if (param_type != param->type)
 		return -EINVAL;
-	err = devlink_param_value_get_from_info(param, info, &value);
-	if (err)
-		return err;
-	if (param->validate) {
-		err = param->validate(devlink, param->id, value, info->extack);
+
+	reset_default = info->attrs[DEVLINK_ATTR_PARAM_RESET_DEFAULT];
+	if (!reset_default) {
+		err = devlink_param_value_get_from_info(param, info, &value);
 		if (err)
 			return err;
+		if (param->validate) {
+			err = param->validate(devlink, param->id, value,
+					      info->extack);
+			if (err)
+				return err;
+		}
 	}
 
 	if (GENL_REQ_ATTR_CHECK(info, DEVLINK_ATTR_PARAM_VALUE_CMODE))
···
 		return -EOPNOTSUPP;
 
 	if (cmode == DEVLINK_PARAM_CMODE_DRIVERINIT) {
+		if (reset_default) {
+			if (!param_item->driverinit_value_valid) {
+				NL_SET_ERR_MSG(info->extack,
+					       "Default value not available");
+				return -EOPNOTSUPP;
+			}
+			value = param_item->driverinit_default;
+		}
+
 		param_item->driverinit_value_new = value;
 		param_item->driverinit_value_new_valid = true;
 	} else {
···
 			return -EOPNOTSUPP;
 		ctx.val = value;
 		ctx.cmode = cmode;
-		err = devlink_param_set(devlink, param, &ctx, info->extack);
+		if (reset_default)
+			err = devlink_param_reset_default(devlink, param, cmode,
+							  info->extack);
+		else
+			err = devlink_param_set(devlink, param, &ctx,
+						info->extack);
 		if (err)
 			return err;
 	}
···
 
 	param_item->driverinit_value = init_val;
 	param_item->driverinit_value_valid = true;
+	param_item->driverinit_default = init_val;
 
 	devlink_param_notify(devlink, 0, param_item, DEVLINK_CMD_PARAM_NEW);
 }