LFSR_TBLSIZE
coarse >= COARSE_DIV_OFFSET + LFSR_TBLSIZE;
unsigned long lo = req->best_parent_rate / (COARSE_DIV_OFFSET + LFSR_TBLSIZE) + 1;
u16 fwd[LFSR_TBLSIZE];
u16 inv[LFSR_TBLSIZE];