CLK_TBG_SEL_SHIFT
prev_clock_idx = (val >> CLK_TBG_SEL_SHIFT) & CLK_TBG_SEL_MASK;
val &= ~(CLK_TBG_SEL_MASK << CLK_TBG_SEL_SHIFT);
val |= parent_clock_idx << CLK_TBG_SEL_SHIFT;