FreeRTOS's Xtensa port, the Xtensa processor support and newlib's Xtensa memory and string functions included in ChippyOS.
They carry the license below.

The port and interrupt code:

Copyright (c) 2015-2019 Cadence Design Systems, Inc.

The Xtensa processor support:

Copyright (c) 1998-2002 Tensilica Inc.
Copyright (c) 1999-2021 Tensilica Inc.
Copyright (c) 2000-2010 Tensilica Inc.
Copyright (c) 2000-2014 Tensilica Inc.
Copyright (c) 2002-2012 Tensilica Inc.
Copyright (c) 2002-2013 Tensilica Inc.
Copyright (c) 2005-2011 Tensilica Inc.
Copyright (c) 2006-2018 Tensilica Inc.
Copyright (c) 2012-2013 Tensilica Inc.
Copyright (c) 1999-2015 Cadence Design Systems, Inc.
Copyright (c) 1999-2021 Cadence Design Systems Inc.
Copyright (c) 2005-2015 Cadence Design Systems, Inc.
Copyright (c) 2006-2015 Cadence Design Systems Inc.

The Xtensa HAL library (libxt_hal), as ESP-IDF's list of copyrights gives it:

Copyright (C) 2003, 2006, 2010 Tensilica Inc.

newlib's Xtensa memory and string functions (memcpy, memset, strcmp, strcpy, strlen and strncpy) and the header they share:

Copyright (c) 2002-2008 Tensilica Inc.
Copyright (c) 2001-2008 Tensilica Inc.
Copyright (c) 2001-20012 Tensilica Inc.
Copyright (c) 2006 Tensilica Inc.

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.