The usbChooseChannel function is responsible for finding the channel that should be used to contain a sequence of events for a particular tool/contact. If a tool/contact is already using a particular channel (i.e., the last event in the channel has the correct device type, serial number, and is in proximity) then usbChooseChannel will return that channel. Otherwise, usbChooseChannel will try to find a free channel (i.e., one which is out of proximity) to be used to store the tool's sequence of events.
The existing logic has a subtle bug that may result in a malfunction when a specific condition occurs. In particular, if a multitouch report from the kernel contains an existing contact going up followed by a brand new contact, then usbChooseChannel will see the channel associated with the former contact as free and begin using it to store data for the new contact. As a result, the "up" event is never sent and can lead to further strange touch behavior (including the effective disabling of touch input). To fix this, we modify usbChooseChannel to consider the "valid" state as well as the working state when finding a new channel. A channel should not be considered free if its valid state is still in prox, since this indicates that the channel has *just* left prox. Note that we cannot rely on only checking the valid state since this fails in the opposite way: usbChooseChannel would not notice when a channel has *just* entered prox and would overwrite its contents. It isn't obvious wether it is also necessary to check the valid state for the other two cases in this function (finding an existing channel and forcefully clearing channels when out of space). The existing code seems to work, however, so we will leave it as-is. Ref: https://sourceforge.net/p/linuxwacom/bugs/335/?limit=25&page=1#73fe Ref: https://sourceforge.net/p/linuxwacom/bugs/339/#f940 Ref: https://sourceforge.net/p/linuxwacom/bugs/349/ Signed-off-by: Jason Gerecke <jason.gere...@wacom.com> --- src/wcmUSB.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/src/wcmUSB.c b/src/wcmUSB.c index ca63e6d..8ded958 100644 --- a/src/wcmUSB.c +++ b/src/wcmUSB.c @@ -932,7 +932,8 @@ static int usbChooseChannel(WacomCommonPtr common, int device_type, unsigned int if (i == PAD_CHANNEL) continue; - if (!common->wcmChannel[i].work.proximity) + if (!common->wcmChannel[i].work.proximity && + !common->wcmChannel[i].valid.state.proximity) { channel = i; memset(&common->wcmChannel[channel],0, sizeof(WacomChannel)); -- 2.16.2 ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Linuxwacom-devel mailing list Linuxwacom-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linuxwacom-devel
