  Luminous quasars powered by accreting supermassive black holes (SMBHs) have
been found in the early Universe at $z \gtrsim 7.5$, which set a strong
constraint on both the seed black hole mass and the rapid growth of the SMBHs.
In this work, we explore how the SMBHs are grown through Eddington limited
accretion driven predominantly by magnetic outflows. Most angular momentum and
the released gravitational energy in the disk can be removed by magnetic
outflows, and therefore the mass accretion rate of the black hole (BH) can be
high even if the disk is radiating at sub-Eddington luminosity. It is found
that the SMBH with several billion solar masses discovered at $z\gtrsim 7$ may
probably be grown through chaotic accretion predominantly driven by magnetic
outflows from a stellar mass BH, when the disks are radiating at moderate
luminosity ($\sim 0.5$ Eddington luminosity) with mild outflows. We find that
most SMBHs are spinning at moderate values of spin parameter $a_*$, which
implies only a small fraction of quasars may have radio jets.
