The SPDK repo contains multiple git submodules each with its own
license info.

Submodule license info:
dpdk: see dpdk/license
intel-ipsec-mb: see intel-ipsec-mb/LICENSE
isa-l: see isa-l/LICENSE
libvfio-user: see libvfio-user/LICENSE
ocf: see ocf/LICENSE

The rest of the SPDK repository uses the Open Source BSD-3-Clause
license.  SPDK also uses SPDX Unique License Identifiers to eliminate
the need to copy the license text into each individual file.

Any new file contributions to SPDK shall adhere to the BSD-3-Clause
license and use SPDX identifiers. Exceptions are subject to usual
review and must be listed in this file.

Exceptions:

* include/linux/* header files are BSD-3-Clause but do not use SPDX
  identifier to keep them identical to the same header files in the
  Linux kernel source tree.

* include/spdk/tree.h and include/spdk/queue_extras are BSD-2-Clause,
  since there were primarily imported from FreeBSD.  tree.h uses an SPDX
  identifier but also the license text to reduce differences from the
  FreeBSD source tree.

* lib/util/base64_neon.c is BSD-2-Clause.