The r316585 did break the sparc build, as the current code does not implement method to detect the disk size, The userboot does query for disk size, but we need to update ioctl method and provide callback for vdev_probe().
The OFW actually does define methods:
block-size - to get the media sector size
#blocks (and/or #blocks64) to get the disk size.
However, even as I wrote the wrappers for the methods, I have no way to test the implementations, also the disk size itself does not help us as the ofw code does not really use common/disk.c API and therefore the those functions are likely useful in future work, but not right now.
The solution to provide partition size for vdev_probe() is about making partition size info available, For now, I am providing the partition size via global variable, as we call zfs_dev_probe() for each (recognized) partition.
For userboot, the fix is quite straight forward, as userboot is already using common/disk.c API.