The Windows Precision Touchpad spec "Figure 4 Button Only Down and Up"
and "Table 9 Report Sequence for Button Only Down and Up" indicate
that the first packet of a (possibly hybrid mode multi-packet) report
may contain a contact-count of 0 if only a button is pressed and no
fingers are detected.
This means that a value of 0 for contact-count is a valid value and
should be used as expected contact count when num_received == 0.
The only other case where we may get a contact count value of 0
when num_received == 0 is when we're somehow out of sync and in
that case setting num_expected to 0 so that we ignore contacts
until we're back in sync again also is the right thing to do.
Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
---
drivers/hid/hid-multitouch.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
index bb939f6990f1..76a3e9e074ba 100644
--- a/drivers/hid/hid-multitouch.c
+++ b/drivers/hid/hid-multitouch.c
@@ -827,7 +827,7 @@ static void mt_touch_report(struct hid_device *hid, struct hid_report *report)
if (td->cc_index >= 0) {
struct hid_field *field = report->field[td->cc_index];
int value = field->value[td->cc_value_index];
- if (value)
+ if (value || td->num_received == 0)
td->num_expected = value;
}
--
2.14.3
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