The set_dataplane() API in <sys/dataplane.h> originally supported the
Tilera version of Linux as shipped to our customers. Once we started
upstreaming the dataplane support in the kernel, the API changed
to use fcntl() as part of the current task-isolation patch series.
It doesn't seem like continuing to support the old API is useful
for newly-compiled code, and even supporting the old glibc binary
API on an upstream kernel that supports the new task isolation mode
isn't straightforward, since the semantics have changed in ways that
make it hard to map the old semantics precisely to the new ones,
so just return ENOSYS.
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