Full docs can be found on docs.rs.
Most of DPI is licensed under the Apache License, Version 2.0 (LICENSE).
All files except for src/libm.rs (and LICENSE-LIBM-MIT) are available solely under that license.
For its no_std support, DPI uses code from the libm crate.
This is in the libm.rs file, and is licensed solely under the MIT Licence (LICENSE-LIBM-MIT).
That file contains details of all potentially applicable copyright notices.
This is feature gated to only be included if you disable the std feature, otherwise it will not be compiled into your final binary
(and so these license terms will not apply).
Overall, this means that the license for this crate depends on what features you have enabled.
If you enable the std feature, then DPI uses only code available under the Apache-2.0 license, and so can be used under the terms of that license.
However, if you disable the std feature, then both these licenses must be followed to use the crate as a whole.