sdhci - Implement ADMA2 transfer support. Keep SDMA support for now.
* Using ADMA2 transfers, only generates 1 or 2 interrupts for each
bio transfer, whereas the existing SDMA code was only transferring
in 4K blocks (with a completion interrupt for each 4K block).
* Since support for UHS high-speed modes is still missing this might
only give a small performance increase in many cases.
With very aggressive CPU C-state power-management and/or very slow
CPUs, this can improve performance a lot however. e.g. on an Atom
x5-Z8300 at 480MHz using deepest mwait c-state setting:
- eMMC read speed: ca. 17 MiB/s --> 30 MiB/s
- SD-card read speed: ca. 11MiB/s --> 18MiB/s