Theo de Raadt [deraadt@openbsd.org] wrote:
> 
> The diff in question which adds BUS_DMA_64BIT is a declaration to
> the higher-level code that _all_ chipsets supported by this driver can
> work with 64 bit-addressable memory if the higher levels provide it.
> 

Linux iwlwifi has a 36-bit dma limit for older hardware. For the
AX200/201 and the AX210/211 it's 64-bit all the way. So iwx is
doing the right thing.

However, there is a bug with the AX200/201 dma engine crossing 32-bit
boundaries:

https://code.opensuse.org/kernel/kernel-source/blob/5377a688dcc5c526f0a5df6d91df22d8b4f2dce6/f/patches.suse/iwlwifi-pcie-extend-hardware-workaround-to-context-i.patch

To avoid triggering it, we'd need to do something like this with
any post-IOMMU addresses:

iwx_crosses_4g_boundary(struct iwx_softc *sc, bus_addr_t addr, bus_size_t len) {
	if (sc->sc_device_family != IWX_DEVICE_FAMILY_22000)
		return 0;

	return (addr >> 32) != ((addr + len) >> 32);
}
...
	for (i = 0; i < map->dm_nsegs; i++) {
		if (iwx_crosses_4g_boundary(sc, map->dm_segs[i].ds_addr,
		    map->dm_segs[i].ds_len)) {
			/* remap */
		}
	}