How do clocks work on the BBB in the DTS?

GoogleGroups
1

I see:

  sys_clkin_ck: sys_clkin_ck {
    #clock-cells = <0>;
    compatible = "ti,mux-clock";
    clocks = <&virt_19200000_ck>, <&virt_24000000_ck>, <&virt_25000000_ck>, <&virt_26000000_ck>;
    ti,bit-shift = <22>;
    reg = <0x0040>;
  };

  mcasp0_fck: mcasp0_fck {
    #clock-cells = <0>;
    compatible = "fixed-factor-clock";
    clocks = <&sys_clkin_ck>;
    clock-mult = <1>;
    clock-div = <1>;
  };

But I can't tell which clock ends up feeding the mcasp0. Thanks!