A common problem when debugging a driver is retrieving the soft state for a particular driver instance. The soft state is allocated with the ddi_soft_state_zalloc(9F) routine. The driver can obtain the soft state through ddi_get_soft_state(9F). The name of the soft state pointer is the first argument to ddi_soft_state_init(9F)). With the name, you can use mdb to retrieve the soft state for a particular driver instance through the ::softstate dcmd:
> *bst_state::softstate 0x3 702b7578
In this case, ::softstate is used to fetch the soft state for instance 3 of the bst sample driver. This pointer references a bst_soft structure that is used by the driver to track state for this instance.