It's pretty obsecure and confused me for a long time so I figured it's
worth documenting properly.
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200414233502.758-1-oohall@gmail.com