msm_fb data readout, writeback panel readout

Hello, how do I read out the data currently on a display?  My device is
a msm-based Android phone, the device nodes are at /dev/graphics/fb{0,1} using
msm_fb. The fb1 device is a "writeback panel" which in my understanding
maintains a readable buffer meant for things like screen capture.

I can't find any information on getting data from the buffer, and was informed
there may be compression getting in the way. I found reference to the flag to
turn it off, but would prefer to make use of it. Can anyone help?

Thanks in advance!