devlink: allow to call devl_param_driverinit_value_get() without holding instance lock

If the driver maintains following basic sane behavior, the
devl_param_driverinit_value_get() function could be called without
holding instance lock:

1) Driver ensures a call to devl_param_driverinit_value_get() cannot
   race with registering/unregistering the parameter with
   the same parameter ID.
2) Driver ensures a call to devl_param_driverinit_value_get() cannot
   race with devl_param_driverinit_value_set() call with
   the same parameter ID.
3) Driver ensures a call to devl_param_driverinit_value_get() cannot
   race with reload operation.

By the nature of params usage, these requirements should be
trivially achievable. If the driver for some off reason
is not able to comply, it has to take the devlink->lock while
calling devl_param_driverinit_value_get().

Remove the lock assertion and add comment describing
the locking requirements.

This fixes a splat in mlx5 driver introduced by the commit
referenced in the "Fixes" tag.

Lore: https://lore.kernel.org/netdev/[email protected]/
Reported-by: Kim Phillips <[email protected]>
Fixes: 075935f ("devlink: protect devlink param list by instance lock")
Signed-off-by: Jiri Pirko <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Acked-by: Jakub Kicinski <[email protected]>
Reviewed-by: Jacob Keller <[email protected]>
Tested-by: Kim Phillips <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Jiri Pirko

net/devlink/leftover.c

@@ -9627,14 +9627,23 @@ EXPORT_SYMBOL_GPL(devlink_params_unregister);
  *
  *	This function should be used by the driver to get driverinit
  *	configuration for initialization after reload command.
+ *
+ *	Note that lockless call of this function relies on the
+ *	driver to maintain following basic sane behavior:
+ *	1) Driver ensures a call to this function cannot race with
+ *	   registering/unregistering the parameter with the same parameter ID.
+ *	2) Driver ensures a call to this function cannot race with
+ *	   devl_param_driverinit_value_set() call with the same parameter ID.
+ *	3) Driver ensures a call to this function cannot race with
+ *	   reload operation.
+ *	If the driver is not able to comply, it has to take the devlink->lock
+ *	while calling this.
  */
 int devl_param_driverinit_value_get(struct devlink *devlink, u32 param_id,
 				    union devlink_param_value *val)
 {
 	struct devlink_param_item *param_item;
 
-	lockdep_assert_held(&devlink->lock);
-
 	if (WARN_ON(!devlink_reload_supported(devlink->ops)))
 		return -EOPNOTSUPP;