[Bug 222256] RPI random number generator underruns when BCM2835_RNG_USE_CALLOUT is set

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222256

--- Comment #2 from Stephen J. Kiernan <stevek at freebsd.org> ---
Interesting. The RPi3 must be a bit different than the RPi2 (where the initial
testing using callout was done.) It might make sense to have a sysctl in the
case where one would want to use the callout version to be able to tweak it as
appropriate.

However, it's better to use the default interrupt method rather than the
callout, because, as you saw, there can be cases where the random bit generator
(RBG) cannot provide enough bits to satisfy a request, if the callout period is
too short. With the interrupt method, the RBG is programmed to raise an
interrupt when the FIFO is full. When that occurs, the interrupt handler reads
from the FIFO and feeds the bits read into the random_harvest_queue function.

However, that requires the FDT for the board to have a proper entry for the RBG
with an interrupt attribute, which I don't think the RPi3 FDT has currently.

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