At the PCI root device a 4GB boundary is currently enforced on DMA segments. A change introduced in r232403. This boundary doesn't arise from a specification requirement, rather to preempt any issues with devices which are not capable of encountering a 4GB boundary.
Unfortunately enforcing an artificial boundary is an imposition for another devices. For example the ioat(4) driver when it used in conjunction with ntb(4). Given the larger memory sizes available today it seems reasonable remove the general restriction and handle it on a case-by-case basis as needed.
For the few devices with known 4GB DMA boundary requirements, which were removed after r232403, go ahead and restore them. As suggested by jhb@ these include r232669 for twa(4) and r232766 for bge(4).