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By counting the number of times we have woken up, we have a very simple means of defining an epoch, which will come in handy if we want to perform deferred tasks at the end of an epoch (i.e. while we are going to sleep) without imposing on the next activity cycle. v2: No reason to specify precise number of bits here. v3: Take Tvrtko's advice and reserve 0 as an invalid epoch. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20180124113608.14909-1-chris@chris-wilson.co.uk