  The existence of $\gtrsim10^9M_\odot$ supermassive black holes (SMBHs) at
redshift $z>6$ raises the problem of how such SMBHs can grow up within the
cosmic time ($<1$\,Gyr) from small seed BHs. In this letter, we use the
observations of $14$ Quasars at $z>6.5$ with mass estimates to constrain their
seeds and early growth, by self-consistently considering the spin evolution and
the possibility of super-Eddington accretion. We find that spin plays an
important role in the growth of early SMBHs, and the constraints on seed mass
and super-Eddington accretion fraction strongly depend on the assumed accretion
history. If the accretion is coherent with single (or a small number of)
episode(s), leading to high spins for the majority of accretion time, then the
SMBH growth is relatively slow; and if the accretion is chaotic with many
episodes and in each episode the total accreted mass is much less than the SMBH
mass, leading to moderate/low spins, then the growth is relatively fast. The
constraints on the seed mass and super-Eddington accretion fraction are
degenerate. A significant fraction ($\gtrsim0.1\%-1\%$ in linear scale but
$\sim 3-4$ dex in logarithmic scale for $10^3-10^4 M_\odot$ seeds) of
super-Eddington accretion is required if the seed mass is not
$\gg10^{5}M_\odot$, and the requirements of high seed-mass and/or
super-Eddington accretion fraction are moderately relaxed if the accretion is
chaotic.
