Xenomai 2 to 3 migration says "RTDM drivers implementing differentiated ioctl() support for both domains should serve all real-time only requests from ioctl_rt(), returning -ENOSYS for any unrecognized request, which will cause the adaptive switch to take place automatically to the ioctl_nrt() handler. The ioctl_nrt() should then implement all requests which may be valid from the regular Linux domain exclusively."

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ioctl.c