power_check_constraints
if (power_check_constraints(cpuhw, cpuhw->events, cpuhw->flags, n0 + 1, cpuhw->event))
i = power_check_constraints(cpuhw, cpuhw->events, cpuhw->flags, n, cpuhw->event);
err = power_check_constraints(cpuhw, events, cflags, n + 1, ctrs);