cpu_down
int (*cpu_down)(struct mips_cdmm_device *);
BUILD_PERDEV_HELPER(cpu_down) /* int mips_cdmm_cpu_down_helper(...) */
.cpu_down = mips_ejtag_fdc_tty_cpu_down,
return cpu_down(dev->id, CPUHP_OFFLINE);
ret = cpu_down(dev->id, target);