int bus_for_each_dev (struct bus_type * bus, struct device * start, void * data, int (*fn) struct device *, void *);
Iterate over bus's list of devices, and call fn for each, passing it data. If start is not NULL, we use that device to begin iterating from.
We check the return of fn each time. If it returns anything other than 0, we break out and return that value.
The device that returns a non-zero value is not retained in any way, nor is its refcount incremented. If the caller needs to retain this data, it should do, and increment the reference count in the supplied callback.