On Tue, Mar 12, 2013 at 02:10:26PM -0400, Yufeng Shen wrote: > On Mon, Mar 11, 2013 at 9:36 PM, Peter Hutterer > <peter.hutterer@xxxxxxxxx> wrote: > > > > On Mon, Mar 11, 2013 at 04:34:27PM -0400, Yufeng Shen wrote: > > > I have ran into cases where I want to make a touch end event to have a > > > touch cancel indication. > > > > > > This comes from trying to solve the problem of : > > > > > > If the touch sequence happens before the system suspends, and the touch > > > release event is > > > never received after the system resumes, userspace MT state tracking could > > > be in a bad state. > > > > > > ( see #5 from > > > https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-input-synaptics/+bug/968845 > > > for an example of how this could happen from lid close/open on MBA) > > > > ftr, this a bug in the driver and should be fixed now. > > By driver do you mean the trackpad driver or the xf86-input driver ? xf86-input-synaptics > From the thread > I can only see the fix in xf85-input-synaptics to clear the touch > device HW state on > resume, which I would expect to have the same side effect of triggering unwanted > gesture that I want address in this post, right ? > > If there is a fix in trackpad kernel driver, can you point me to that ? there are two issues: one was that the driver never cleared the state correctly and thus was left with inconsistent state. this caused a lot of stuck button bugs, etc. that part was buggy and got fixed in xf86-input-synaptics. the server also releases all buttons/touches on a device when it gets disabled to avoid stuck buttons. this should release any touch sequences from a client's POV. the issue you want to fix is that you don't want to send those events but something else to avoid the clients interpreting this as a release. correct? what's the exact use-case here, what's your client stack, how often does this happen? > > > One possible workaround is to let the touch device driver to release all > > > existing touches on > > > resume, which has the effect of clearing all the MT states in userspace > > > touch stacks. > > > But the touch release/end event often will result in some gesture being > > > recognized and performed, > > > like a tap-to-click being generated. > > > > > > So I am wondering what's the best way to solve the problem of clearing the > > > touch states with > > > minimal side effect. One way I can think of is to have MTB protocol add > > > support of > > > a touch cancel indication on touch release, e.g. making TRACKING_ID = -2 > > > meaning that > > > the touch release is synthesized from the system and really has the meaning > > > of releasing and canceling the current touch, while TRACKING_ID = -1 > > > meaning that the touch release is reported back from the device. > > > > > > And from Xf86-input driver level, we can add a corresponding TouchCancel > > > for this. > > > > I can handle touch-cancel events in the synaptics driver to avoid > > tap-to-click but further details get a bit nasty. > > > > To actually add TouchCancel to the client-protocol means a new XI protocol > > revision, plus the stuff in the server _and_ the stuff in the client. that > > is quite some lag time here, and if a client cannot handle TouchCancel all > > we can do is do a TouchEnd - which will still trigger the gesture. > > > > even if you update the touch clients you're still lacking any solution for > > pointer-emulated clients. again, here we can only do a ButtonRelease event > > which again will trigger whatever it did. > > > > All the above can be implemented though. In fact, I suspect the protocol > > part is the easy bit (just a flag on TouchEnd) but the server part is > > reasonably nasty. > > > > the real counter-argument is that I think it is a partial solution only. > > From an X perspective touches also end when you vt-switch away from the > > server (device is disabled). but the kernel won't cancel the touch event for > > that. Or when the device is disabled by the client ("disable touchpad while > > typing" feature), So we'd have to maintain both implicit cancel and explicit > > cancel in the driver anyway. > > > > so yeah, I don't think adding this to the kernel would provide any > > significant benefit since we still need to handle all the other cases > > anyway. > > > > What's your suggestion on how to handle all the other cases ? > > > If you feel it is too much effort to take in X server/client code just for > fixing this particular case, how about a more light weighted solution: > we still need kernel to mark some touch end event as synthesized, and > in xf86-input driver (e.g. evdev), we attach the XI_TouchEnd event with > a special valuator to indicate it has meaning of touch cancel. Then for > X clients who wants to handle this case, they can check this particular > valuator. no, i'm not putting hacks like this into the protocol. first, whether the kernel does this or not is largely irrelevant to the X driver since the kernel suspend is only one case of disabling the device. VT-switch and disabling the device manually are the other two and the kernel won't (and can't) give us events for this. second, the only way you can handle such events in the client is to add it to XI 2.4, i.e. a proper protocol extension with the spec to describe how and when it is generated, what the effects on grabs and pointer emulation is, etc. the easiest solution is to add a TouchCancelled flag to the TouchEnd event, since that flag must be ignored by clients < XI 2.4 and we don't need to introduce a new event type. so it's largely backwards-compatible, but clients not supporting 2.4 will still release the gesture. That doesn't cover the various details though. you need to also integrate that into the server and I can warn you ahead of time, touch grab ownership handling is pretty insane. Either way, that part is a topic for xorg-devel@xxxxxxxxxxx. Cheers, Peter -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
