Reading ADC, get "cat: in_voltage0_raw: Device or resource busy"

What am I missing here?

# uname -r
3.8.13-bone72

# pwd
/sys/bus/iio/devices/iio:device0

# ls
dev in_voltage0_raw in_voltage1_raw in_voltage2_raw name power/ subsystem@ uevent

# cat in_voltage0_raw
cat: in_voltage0_raw: Device or resource busy

The fragment from my DTBO:

  fragment@6 {
    target = <&tscadc>;
    __overlay__ {

      status = "okay";
      adc {
        ti,adc-channels = <0 1 2>;
      };
    };
  };

Thanks,