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? Thanks. -- 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
