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The `wacom_feature_mapping` function is careful to only set the the
touch_max value a single time, but this care does not extend to the
`wacom_wac_finger_event` function. In particular, if a device sends
multiple HID_DG_CONTACTMAX items in a single feature report, the
driver will end up retaining the value of last item.

The HID descriptor for the Cintiq Companion 2 does exactly this. It
incorrectly sets a "Report Count" of 2, which will cause the driver
to process two HID_DG_CONTACTCOUNT items. The first item has the actual
count, while the second item should have been declared as a constant
zero. The constant zero is the value the driver ends up using, however,
since it is the last HID_DG_CONTACTCOUNT in the report.

    Report ID (16),
    Usage (Contact Count Maximum),  ; Contact count maximum (55h, static value)
    Report Count (2),
    Logical Maximum (10),
    Feature (Variable),

To address this, we add a check that the touch_max is not already set
within the `wacom_wac_finger_event` function that processes the
HID_DG_TOUCHMAX item. We emit a warning if the value is set and ignore
the updated value.

This could potentially cause problems if there is a tablet which has
a similar issue but requires the last item to be used. This is unlikely,
however, since it would have to have a different non-zero value for
HID_DG_CONTACTMAX earlier in the same report, which makes no sense
except in the case of a firmware bug. Note that cases where the
HID_DG_CONTACTMAX items are in different reports is already handled
(and similarly ignored) by `wacom_feature_mapping` as mentioned above.

Link: https://github.com/linuxwacom/input-wacom/issues/223
Fixes: 184eccd4

 ("HID: wacom: generic: read HID_DG_CONTACTMAX from any feature report")
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
CC: stable@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>