FFTW license

[lnicola]

FFTW is licensed under the GPL, which means that this crate should also be GPL, as opposed to the current BSD-3-Clause license. You might want to relicense or switch to the Intel MKL backend of fftw-sys or another, Rust-based, implementation.

[skailasa]

@lnicola , thanks for this. @mscroggs and @tbetcke , this may mean that we will have to update the license on bempp-rs as well? I'm not familiar with the specific differences between GPL and BSD3 however, and what the implications of this would be.

[mscroggs]

Currently, bempp-rs's dependency on kifmm is commented out, but we'll need to think about this. I'd prefer to move to using the Intel MKL backend or another implementation rather than moving to GPL though: the viral-ness of GPL can be a pain. But maybe @tbetcke has other opinions

[skailasa]

The problem with the MKL backend is we are running this code a lot on ARM chips, and there aren't really any suitable Rust implementations.

[lnicola]

Does https://crates.io/crates/rustfft work?

[skailasa]

I've used it in the past - it would be possible to use with a bit of effort - we do 3D FFTs and the convenience wrappers are handy in FFTW. In terms of performance, i've seen the maintainer's posts about competitiveness with FFTW, but FFTW is tried and tested and used by our own competitors so I prefer it to a relatively new project.

[mscroggs]

In the past we had a version of bempp that was MIT licensed or GPL licensed if a dependency was used (iirc it was DUNE's Alugrid in that case). Could be we could do something similar here with the default being MKL with BSD license, or the license being GPL if a feature that uses the ARM fftw is activated

In the sort term, I guess it's best to relicense this to be GPL then move to a longer term solution once we have time to make something else work

[skailasa]

In the past we had a version of bempp that was MIT licensed or GPL licensed if a dependency was used (iirc it was DUNE's Alugrid in that case). Could be we could do something similar here with the default being MKL with BSD license, or the license being GPL if a feature that uses the ARM fftw is activated

In the sort term, I guess it's best to relicense this to be GPL then move to a longer term solution once we have time to make something else work

I was going to ask about something like this, the FFTW dependency is only used in a specific algorithmic variant - so could isolate this as a feature. Wasn't actually sure if you could issue two licenses like this, good to know.

[lnicola]

Wasn't actually sure if you could issue two licenses like this, good to know.

Yeah, it's totally possible. You can OR all of them in the SPDX declaration and document somewhere that feature X changes the license to GPL.

But that native crate would also be interesting, since some users might want to avoid GPL.

[mscroggs]

But that native crate would also be interesting, since some users might want to avoid GPL.

Agreed this seems like an interesting but non-urgent thing to do