Asynchronous function calls
In order to make the kernel boot faster, a set of patches were introduced by Arjan van de Ven in January 2009 to create infrastructure to allow doing some of the initialization steps asynchronously, which will hide significant portions of the hardware delays in practice.
In order to not change device order and other similar observables, the patch does NOT do full parallel initialization.
Rather, it operates more in the way an out of order CPU does; the work may be done out of order and asynchronous, but the observable effects (instruction retiring for the CPU) are still done in the original sequence.
See http://lkml.org/lkml/2009/1/4/155 for the first patch in the series.
Work similar in spirit to this was done previously, but with smaller scope and apparently not mainlined.