  Our charm program uses a mixed action with twisted-mass valence quarks over
non-perturbatively improved Wilson sea quarks, in order to study various
quantities in a relativistic and manifestly local framework of full QCD. The
sea sector consists of $N_\mathrm{f}=2+1$ ensembles generated by the CLS
initiative. Taking advantage of open boundary conditions, this allows access to
fine ensembles without topological freezing. Here we focus in particular on our
current progress on $D\to K\nu l$ and $D\to \pi\nu l$ semileptonics. Those are
first and foremost useful for the computation of the CKM matrix elements
$|V_{cs}|$ and $|V_{cd}|$. We show that all discretisation effects seem to be
reasonably under control with this choice of action, in particular those
related to hypercubic lattice artefacts. Eventually, we obtain preliminary
results of the form factors as a very smooth curve on the whole range of
momentum transfer, and in particular the signal at zero $q^2$ appears to have
the potential to be competitive with earlier published results.
