ivahd high clock

Hi.i need raise ivahd codec rate on omap4460.
i found the ivahd function clock is "dpll_iva_m5x2_ck",but "dpll_iva_m5x2_ck" rate only is 265895798 on omap4460.i want to enhance ivahd high clock.
I try using clk_set_rate() on the IVAHD clock node to change the rate,
such as " clk_set_rate(dpll_iva_m5x2_ck, 372254117);".
then i check the current rate for dpll_iva clock under the /debug/clock,dpll_iva node show rate is 372254117.
But ivahd codec process speed is not enhance.omap4460's(1.2Ghz) ivahd should 420Mhz rate,but now ti publish kernel only 260Mhz.
Can you give me some advision about enhance ducati process speed?