  We describe the first complete polarimetric dataset of the entire outburst of
a low-mass black hole X-ray binary system and discuss the constraints for
geometry and radiative mechanisms it imposes. During the decaying hard state,
when the optical flux is dominated by the non-thermal component, the observed
polarization is consistent with the interstellar values in all filters. During
the soft state, the intrinsic polarization of the source is small, $\sim 0.15$
per cent in $B$ and $V$ filters, and is likely produced in the irradiated disc.
A much higher polarization, reaching $\sim 0.5$ per cent in $V$ and $R$
filters, at position angle of $\sim 25^\circ$ observed in the rising hard state
coincides in time with the detection of winds in the system. This angle
coincides with the position angle of the jet. The detected optical polarization
is best explained by scattering of the non-thermal (hot flow or jet base)
radiation in an equatorial wind.
