On Tue, 16 Feb 2021, Jason Gerecke wrote:

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: 184eccd40389 ("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

---

drivers/hid/wacom_wac.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c index 1bd0eb71559c..44d715c12f6a 100644 --- a/drivers/hid/wacom_wac.c +++ b/drivers/hid/wacom_wac.c @@ -2600,7 +2600,12 @@ static void wacom_wac_finger_event(struct hid_device *hdev, wacom_wac->is_invalid_bt_frame = !value; return; case HID_DG_CONTACTMAX:

• features->touch_max = value;
  

• if (!features->touch_max) {
  

• 	features->touch_max = value;
  

• } else {
  

• 	hid_warn(hdev, "%s: ignoring attempt to overwrite non-zero touch_max "
  

• 		 "%d -> %d\n", __func__, features->touch_max, value);
  

• }
  

  return;

Applied, thanks Jason.