On Wed, Apr 23, 2014 at 03:55:28PM +1000, Peter Hutterer wrote: > On Tue, Apr 22, 2014 at 10:46:47PM -0700, Dmitry Torokhov wrote: > > On Wed, Apr 23, 2014 at 03:38:49PM +1000, Peter Hutterer wrote: > > > On Wed, Apr 23, 2014 at 10:21:03AM +1000, Peter Hutterer wrote: > > > > On Tue, Apr 22, 2014 at 08:21:54AM +0200, David Herrmann wrote: > > > > > Hi Peter > > > > > > > > > > On Tue, Apr 22, 2014 at 6:15 AM, Peter Hutterer > > > > > <peter.hutterer@xxxxxxxxx> wrote: > > > > > > How are you planning to handle the slot-based events? We'd either need to > > > > > > add something similar (but more complex) to evdev_handle_mt_request or rely > > > > > > on the caller to call the whole EV_ABS range and ditch anything ABS_MT_. > > > > > > I'd prefer the former, the latter is yet more behaviour that's easy to get > > > > > > wrong. > > > > > > > > > > This is all racy.. > > > > > > > > > > We _really_ need an ioctl to receive _all_ ABS information atomically. > > > > > I mean, there's no way we can know the user's state from the kernel. > > > > > Even if the user resyncs via EVIOCGMTSLOTS, we can never flush the > > > > > whole ABS queue. Problem is, the user has to call the ioctl for _each_ > > > > > available MT code and events might get queued in between. So yeah, > > > > > this patch doesn't help much.. > > > > > > > > > > I have no better idea than adding a new EVIOCGABS call that retrieves > > > > > ABS values for all slots atomically (and for all other axes..). No > > > > > idea how to properly fix the old ioctls. > > > > > > > > bonus points for making that ioctl fetch the state of the last SYN_DROPPED > > > > and leave the events since in the client buffer. That way we can smooth over > > > > SYN_DROPPED and lose less information. > > > > > > to expand on this, something like the below would work from userspace: > > > > > > 1. userspace opens fd, EVIOCGBIT for everything > > > 2. userspace calls EVIOCGABSATOMIC > > > 3. kernel empties the event queue, flags the client as capable > > > 4. kernel copies current device state into client-specific struct > > > 5. kernel replies with that device state to the ioctl > > > 6. client reads events > > > .. > > > 7. kernel sees a SYN_DROPPED for this client. Takes a snapshot of the device > > > for the client, empties the buffer, leaves SYN_DROPPED in the buffer > > > (current behaviour) > > > 8. client reads SYN_DROPPED, calls EVIOCGABSATOMIC > > > 9. kernel replies with the snapshot state, leaves the event buffer otherwise > > > unmodified > > > 10. client reads all events after SYN_DROPPED, gets a smooth continuation > > > 11. goto 6 > > > > > > if the buffer overflows multiple times, repeat 7 so that the snapshot state > > > is always the last SYN_DROPPED state. well, technically the state should be > > > the state of the device at the first SYN_REPORT after the last SYN_DROPPED, > > > since the current API says that interrupted event is incomplete. > > > > > > there are two oddities here: > > > 1. the first ioctl will have to flush the buffer to guarantee consistent state, > > > though you could even avoid that by taking a snapshot of the device on > > > open(). though that comes with a disadvantage, you don't know if the client > > > supports the new approach so you're wasting effort and memory here. > > > 2. I'm not quite sure how to handle multiple repeated calls short of > > > updating the client-specific snapshot with every event as it is read > > > successfully. > > > > > > any comments? > > > > Do we really need to optimize the case when we are dropping events? > > It happens frequently, to the point where on some laptops you're pretty much > guaranteed to get SYN_DROPPED events on resume and sometimes even during > normal multi-finger user. > > I don't have any measurements on how many events are dropped on average. > Could be one or two, could be several buffer sizes, I honestly don't know. I think we need to figure this out. The idea is that dropping events should be an exception, not a rule. -- Dmitry -- 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
