On Nov 06 2017 or thereabouts, Hans de Goede wrote:
> 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.
Please add something here that explains that num_received == 0 means
that we are at the beginning of a multi-report frame. I had a hard time
remembering that actually.
>
> 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.
Are you sure about that? I mean, if you set num_expected to 0, all
incoming report will call mt_sync_frame(), which basically flushes all
the events to the user space.
I am fine accepting a contact count value of 0 if we are at the
beginning of a multi report frame, but I'd be more happy if you take
into account the Scan Time HID field value into account instead of a
computed value.
Cheers,
Benjamin
>
> 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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