Re: [PATCH] evdev: flush ABS_* events during EVIOCGABS

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

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@who-t.net> 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