  We present the pion mass and decay constant using the overlap fermion valence
on domain wall (DW) fermion sea at several lattice spacings. The mixed action
effect in the lattice calculation is also studied, and the result suggests that
the mixed action effect with the overlap valence on DW sea would be
proportional to the fourth power of the lattice spacing. We obtain the pion
decay constant at the physical pion mass and $N_f=2$ chiral limit to be
$92.4(3)(2)~{\rm MeV}$ and $87.0(5)(7)~{\rm MeV}$ respectively; and the
physical u/d averaged quark mass at $\overline{\textrm{MS}}$ 2 GeV is
$3.74(4)(5)(5)(3)~{\rm MeV}$ with the linear ${\cal O}(a^2)$ continuum
extrapolation. Using the FLAG value of the NLO low energy constant, we obtain
the $N_f=2$ chiral condensate to be $\Sigma^{\overline{\textrm{MS}}(2~{\rm
GeV})}=\big(266(2)(1)~{\rm MeV}\big)^3$.
