def METHOD_NAME(inst, mapping):
    """Define sensor type for eyetrack channels.
    This function can set all eye tracking specific information:
    channel type, unit, eye (and x/y component; only for gaze channels)
    Supported channel types:
    ``'eyegaze'`` and ``'pupil'``
    Supported units:
    ``'au'``, ``'px'``, ``'deg'``, ``'rad'`` (for eyegaze)
    ``'au'``, ``'mm'``, ``'m'`` (for pupil)
    Parameters
    ----------
    inst : instance of Raw, Epochs, or Evoked
        The data instance.
    mapping : dict
        A dictionary mapping a channel to a list/tuple including
        channel type, unit, eye, [and x/y component] (all as str),  e.g.,
        ``{'l_x': ('eyegaze', 'deg', 'left', 'x')}`` or
        ``{'r_pupil': ('pupil', 'au', 'right')}``.
    Returns
    -------
    inst : instance of Raw | Epochs | Evoked
        The instance, modified in place.
    Notes
    -----
    ``inst.set_channel_types()`` to ``'eyegaze'`` or ``'pupil'``
    works as well, but cannot correctly set unit, eye and x/y component.
    Data will be stored in SI units:
    if your data comes in ``deg`` (visual angle) it will be converted to
    ``rad``, if it is in ``mm`` it will be converted to ``m``.
    """
    ch_names = inst.info["ch_names"]
    # allowed
    valid_types = ["eyegaze", "pupil"]  # ch_type
    valid_units = {
        "px": ["px", "pixel"],
        "rad": ["rad", "radian", "radians"],
        "deg": ["deg", "degree", "degrees"],
        "m": ["m", "meter", "meters"],
        "mm": ["mm", "millimeter", "millimeters"],
        "au": [None, "none", "au", "arbitrary"],
    }
    valid_units["all"] = [item for sublist in valid_units.values() for item in sublist]
    valid_eye = {"l": ["left", "l"], "r": ["right", "r"]}
    valid_eye["all"] = [item for sublist in valid_eye.values() for item in sublist]
    valid_xy = {"x": ["x", "h", "horizontal"], "y": ["y", "v", "vertical"]}
    valid_xy["all"] = [item for sublist in valid_xy.values() for item in sublist]
    # loop over channels
    for ch_name, ch_desc in mapping.items():
        if ch_name not in ch_names:
            raise ValueError(
                "This channel name (%s) doesn't exist in " "info." % ch_name
            )
        c_ind = ch_names.index(ch_name)
        # set ch_type and unit
        ch_type = ch_desc[0].lower()
        if ch_type not in valid_types:
            raise ValueError(
                "ch_type must be one of {}. "
                "Got '{}' instead.".format(valid_types, ch_type)
            )
        if ch_type == "eyegaze":
            coil_type = FIFF.FIFFV_COIL_EYETRACK_POS
        elif ch_type == "pupil":
            coil_type = FIFF.FIFFV_COIL_EYETRACK_PUPIL
        inst.info["chs"][c_ind]["coil_type"] = coil_type
        inst.info["chs"][c_ind]["kind"] = FIFF.FIFFV_EYETRACK_CH
        ch_unit = None if (ch_desc[1] is None) else ch_desc[1].lower()
        if ch_unit not in valid_units["all"]:
            raise ValueError(
                "unit must be one of {}. Got '{}' instead.".format(
                    valid_units["all"], ch_unit
                )
            )
        if ch_unit in valid_units["px"]:
            unit_new = FIFF.FIFF_UNIT_PX
        elif ch_unit in valid_units["rad"]:
            unit_new = FIFF.FIFF_UNIT_RAD
        elif ch_unit in valid_units["deg"]:  # convert deg to rad (SI)
            inst = inst.apply_function(_convert_deg_to_rad, picks=ch_name)
            unit_new = FIFF.FIFF_UNIT_RAD
        elif ch_unit in valid_units["m"]:
            unit_new = FIFF.FIFF_UNIT_M
        elif ch_unit in valid_units["mm"]:  # convert mm to m (SI)
            inst = inst.apply_function(_convert_mm_to_m, picks=ch_name)
            unit_new = FIFF.FIFF_UNIT_M
        elif ch_unit in valid_units["au"]:
            unit_new = FIFF.FIFF_UNIT_NONE
        inst.info["chs"][c_ind]["unit"] = unit_new
        # set eye (and x/y-component)
        loc = np.array(
            [
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
                np.nan,
            ]
        )
        ch_eye = ch_desc[2].lower()
        if ch_eye not in valid_eye["all"]:
            raise ValueError(
                "eye must be one of {}. Got '{}' instead.".format(
                    valid_eye["all"], ch_eye
                )
            )
        if ch_eye in valid_eye["l"]:
            loc[3] = -1
        elif ch_eye in valid_eye["r"]:
            loc[3] = 1
        if ch_type == "eyegaze":
            ch_xy = ch_desc[3].lower()
            if ch_xy not in valid_xy["all"]:
                raise ValueError(
                    "x/y must be one of {}. Got '{}' instead.".format(
                        valid_xy["all"], ch_xy
                    )
                )
            if ch_xy in valid_xy["x"]:
                loc[4] = -1
            elif ch_xy in valid_xy["y"]:
                loc[4] = 1
        inst.info["chs"][c_ind]["loc"] = loc
    return inst