Function lsqr_qsm_padded 

pub(crate) fn lsqr_qsm_padded(
    local_field: &[f64],
    mask: &[u8],
    magnitude: Option<&[f64]>,
    grid: &Grid,
    bdir: (f64, f64, f64),
    params: &LsqrQsmParams,
    progress: impl FnMut(usize, usize),
) -> Option<(Grid, [(usize, usize, usize); 3], Vec<f64>)>

The solve on upstream’s grid. Returns the cropped-and-padded grid, its crop box, and χ on it (unmasked), or None for an empty mask.