What functions are missing if ALSA cannot memmap?

I am helping check out a new ALSA driver, and it seems that jackd is not
happy with the capabilities this driver provides:

configuring for 48000Hz, period = 1024 frames (21.3 ms), buffer = 2 periods
ALSA: mmap-based access is not possible for the capture stream of this
audio interface
ALSA: cannot configure capture channel

Not just capture:
configuring for 48000Hz, period = 1024 frames (21.3 ms), buffer = 2 periods
ALSA: mmap-based access is not possible for the playback stream of this
audio interface
ALSA: cannot configure playback channel

I am not very familiar yet with low level ALSA drivers, any tips where I
should look to see what the driver messed up? Is mmap access usually a
requirement for ALSA drivers?  Is it technically optional but highly
recommended, or it is required and this driver is just incomplete?

-- 
Chris Caudle