From both technical and economical perspectives, all-dc offshore wind farm (OWF) is a trend for future offshore wind energy applications. Furthermore, in an all-dc OWF, both the capacitors in dc collection grid and offshore wind turbines have the potential for onshore grid frequency response. In light of this, this paper presents a model predictive control scheme, which coordinates the operation of offshore dc collection grid capacitors (dc-grid capacitors) and offshore wind turbines for the onshore frequency support. With this scheme, offshore dc-grid capacitors provide fast inertia support right after the frequency event, while offshore wind turbines provide primary frequency response for a longer period. A key feature of the proposed scheme lies in the ability to suppress the secondary frequency drop issue, which is caused by sudden power reduction of offshore dc-grid capacitors during the frequency recovery phase. An analytical framework is derived to validate this ability. Finally, simulation results verify the effectiveness of the proposed scheme.