How to get the correct buffer of the track when input and output port have different channel numbers

Audio Plugins
1

I have an upmix plugin from stereo to 5.1 surround, and it works perfectly on 5.1 track with stereo audio. But the “buffer” parameter in processBlock() always returns 2 channels when I create a stereo track and change the output port into 5.1. Is there a way to get 6-channel buffer in the processBlock() main loop so I can write data into the 6-channel buffer and hear 5.1 sound?

P.S.

  1. I just wanna focus on the case of stereo track with 5.1 output port, because some other products like NUGEN Halo Upmix have made it, and those plugins can process audio differently as the channel number of output port changes.
  2. The 5.1 output port of the track has mapped to physical channels of a multichannel soundcard.