Howto switch dai link from pcm interface to i2s interface

HI all, i am using my mt7621 openwrt 19.07.3 board for sound capturing with wm8974。currently mt
7621 is interfacing with wm8974 by PCM interface , but somthing seemes not good ,So i wanna try I2S interfac , i.e, the cpu dai should be set to ralink-i2s . But the question is , which part of openwrt should be modified ?currently, I cannot see the sound {} node in mt7621.dtsi or MT7621.dts. And iam sure the system is using a simple-card.c file as machine driver. The most confusing thing is : How the system configure the dai link as (ralink -pcm-wm8974)?Where can i find the configuration file or something like that?
I cannot find any dai link thing about mt7621 IC. No sound{} node in the dts file , No files such as :/soc/samsung/s3c24xx_uda134x.c to specify the dai link in sound/soc/ralink dir. So how could simple-card.c know that the codec is connected with pcm interface?